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Ovid: Clinical Neuroanatomy

Authors: Snell, Richard S. Title: Clinical Neuroanatomy, 7th Edition Copyright ©2010 Lippincott Williams & Wilkins > Table of Contents > Chapter 5 – The Brainstem Chapter 5 The Brainstem A 58-year-old woman was referred to a neurologist because of recent onset of difficulty with walking. It was noted that she stood and walked with her left arm flexed at the elbow and the left leg extended (left hemiparesis). While walking, the patient had difficulty flexing the left hip and knee and dorsiflexing the ankle; the forward motion was possible by swinging the left leg outward at the hip to avoid dragging the foot on the ground. The left arm remained motionless. Neurologic examination showed no signs of facial paralysis, but there was evidence of weakness of the movements of the tongue. On protrusion, the tongue deviated toward the right side (right hypoglossal nerve palsy). Cutaneous sensations were found to be normal, but there was evidence of impairment of muscle joint sense, tactile discrimination, and vibratory sense on the left side of the body. Based on the neurologic findings, a diagnosis of right-sided medial medullary syndrome was made. The medial part of the right side of the medulla oblongata receives its arterial supply from the right vertebral artery. Occlusion of this artery or its branch to the medulla results in destruction of the right pyramid (left hemiparesis), destruction of the right hypoglossal nucleus and nerve (right hypoglossal palsy), and destruction of the medial lemniscus on the right side (left-sided loss of muscle joint sense, vibratory sense, and tactile discrimination). The absence of facial palsy showed that the facial nerve nuclei, the facial nerves, and the corticobulbar fibers to the facial nuclei were intact. The sparing of the sensations of touch, pain, and temperature showed that the spinal lemniscus was intact. This diagnosis was possible as the result of carefully sorting out the neurologic findings. A clear knowledge of the position and function of the various nerve tracts and nuclei in the medulla oblongata is essential before one can reach a diagnosis in this case. P.187 Chapter Objectives

  • To review the anatomy of the brainstem
  • To develop a three-dimensional picture of the interior of the brainstem
  • To know the positions of several of the cranial nerve nuclei, the olivary nuclear complex, and the paths taken by the various ascending and descending nerve tracts as they ascend to the higher brain centers or descend to the spinal cord
  • To assess the signs and symptoms presented by a patient and identify the exact location of a structural lesion

Head injuries from blunt trauma and penetrating missiles are associated with a high mortality and disabling morbidity. Because of the close relationship that exists between the skull and the underlying brain and cranial nerves, as well as their common involvement in many diseases, a brief review of the anatomy of the skull will first be considered. A Brief Review of the Skull Adult Skull Composition The skull is composed of several separate bones united at immobile joints called sutures. The connective tissue between the bones is called a sutural ligament. The mandible is an exception to this rule, for it is united to the skull by the mobile temporomandibular joint. The bones of the skull can be divided into those of the cranium and those of the face. The vault is the upper part of the cranium, and the base of the skull is the lowest part of the cranium (Fig. 5-1). The skull bones are made up of external and internal tables of compact bone separated by a layer of spongy bone called the diploë (Fig. 5-2). The internal table is thinner and more brittle than the external table. The bones are covered on the outer and inner surfaces with periosteum. The cranium consists of the following bones, two of which are paired (Figs. 5-1 and 5-3):

• Frontal bone 1
• Parietal bones 2
• Occipital bone 1
• Temporal bones 2
• Sphenoid bone 1
• Ethmoid bone 1

The facial bones consist of the following, two of which are single:

• Zygomatic bones 2
• Maxillae 2
• Nasal bones 2
• Lacrimal bones 2
• Vomer 1
• Palatine bones 2
• Inferior conchae 2
• Mandible 1

It is unnecessary for students to know the detailed structure of each individual skull bone. However, they should be familiar with the skull as a whole, and it would be helpful to have a dried skull available for reference as they read the following description. Anterior View of the Skull The frontal bone, or forehead bone, curves downward to make the upper margins of the orbits (Fig. 5-1). The superciliary arches can be seen on either side, and the supraorbital notch, or foramen, can be recognized. Medially, the frontal bone articulates with the frontal processes of the maxillae and with the nasal bones. Laterally, the frontal bone articulates with the zygomatic bone. The orbital margins are bounded by the frontal bone superiorly, the zygomatic bone laterally, the maxilla inferiorly, and the processes of the maxilla and frontal bone medially. Within the frontal bone, just above the orbital margins, are two hollow spaces lined with mucous membrane called the frontal air sinuses. These communicate with the nose and serve as voice resonators. The two nasal bones form the bridge of the nose. Their lower borders, with the maxillae, make the anterior nasal aperture. The nasal cavity is divided into two by the bony nasal septum, which is largely formed by the vomer. The superior and middle conchae are shelves of bone that project into the nasal cavity from the ethmoid on each side; the inferior conchae are separate bones. The two maxillae form the upper jaw, the anterior part of the hard palate, part of the lateral walls of the nasal cavities, and part of the floors of the orbital cavities. The two bones meet in the midline at the intermaxillary suture and form the lower margin of the nasal aperture. Below the orbit, the maxilla is perforated by the infraorbital foramen. The alveolar process projects downward and, together with the fellow of the opposite side, forms the alveolar arch, which carries the upper teeth. Within each maxilla is a large, pyramid-shaped cavity lined with mucous membrane called the maxillary sinus. This communicates with the nasal cavity and serves as a voice resonator. The zygomatic bone forms the prominence of the cheek and part of the lateral wall and floor of the orbital cavity. It articulates with the maxilla medially and with the zygomatic process of the temporal bone laterally to form the zygomatic arch. The zygomatic bone is perforated by two foramina for the zygomaticofacial and zygomaticotemporal nerves. The mandible, or lower jaw, consists of a horizontal body and two vertical rami. P.188

Figure 5-1 Bones of the anterior aspect of the skull.

Lateral View of the Skull The frontal bone forms the anterior part of the side of the skull and articulates with the parietal bone at the coronal suture (Fig. 5-3). The parietal bones form the sides and roof of the cranium and articulate with each other in the midline at the sagittal suture. They articulate with the occipital bone behind, at the lambdoid suture. The skull is completed at the side by the squamous part of the occipital bone; parts of the temporal bone, namely, the squamous, tympanic, mastoid process, styloid process, and zygomatic process; and the greater wing of the sphenoid. Note the position of the external auditory meatus. The ramus and body of the mandible lie inferiorly. Note that the thinnest part of the lateral wall of the skull is where the anteroinferior corner of the parietal bone articulates with the greater wing of the sphenoid; this point is referred to as the pterion. Clinically, the pterion is an important area because it overlies the anterior division of the middle meningeal artery and vein. Identify the superior and inferior temporal lines, which begin as a single line from the posterior margin of the zygomatic process of the frontal bone and diverge as they arch backward. The temporal fossa lies below the inferior temporal line. The infratemporal fossa lies below the infratemporal crest on the greater wing of the sphenoid. The pterygomaxillary fissure is a vertical fissure that lies within the fossa between the pterygoid process of the sphenoid bone and back of the maxilla. It leads medially into the pterygopalatine fossa. P.189

Figure 5-2 Coronal section of the upper part of the head showing the layers of the scalp, the sagittal suture of the skull, the falx cerebri, the superior and inferior sagittal venous sinuses, the arachnoid granulations, the emissary veins, and the relation of cerebral blood vessels to the subaracnoid space.
Figure 5-3 Bones of the lateral aspect of the skull.

P.190 The inferior orbital fissure is a horizontal fissure between the greater wing of the sphenoid bone and the maxilla. It leads forward into the orbit. The pterygopalatine fossa is a small space behind and below the orbital cavity. It communicates laterally with the infratemporal fossa through the pterygomaxillary fissure, medially with the nasal cavity through the sphenopalatine foramen, superiorly with the skull through the foramen rotundum, and anteriorly with the orbit through the inferior orbital fissure. Posterior View of the Skull The posterior parts of the two parietal bones (Fig. 5-4) with the intervening sagittal suture are seen above. Below, the parietal bones articulate with the squamous part of the occipital bone at the lambdoid suture. On each side, the occipital bone articulates with the temporal bone. In the midline of the occipital bone is a roughened elevation called the external occipital protuberance, which gives attachment to muscles and the ligamentum nuchae. On either side of the protuberance, the superior nuchal lines extend laterally toward the temporal bone. Superior View of the Skull Anteriorly, the frontal bone (Fig. 5-4) articulates with the two parietal bones at the coronal suture. Occasionally, the two halves of the frontal bone fail to fuse, leaving a midline metopic suture. Behind, the two parietal bones articulate in the midline at the sagittal suture. Inferior View of the Skull If the mandible is discarded, the anterior part of this aspect of the skull is seen to be formed by the hard palate (Fig. 5-5). The palatal processes of the maxillae and the horizontal plates of the palatine bones can be identified. In the midline anteriorly is the incisive fossa and foramen. Posterolaterally are the greater and lesser palatine foramina. Above the posterior edge of the hard palate are the choanae (posterior nasal apertures). These are separated from each other by the posterior margin of the vomer and are bounded laterally by the medial pterygoid plates of the sphenoid bone. The inferior end of the medial pterygoid plate is prolonged as a curved spike of bone, the pterygoid hamulus. Posterolateral to the lateral pterygoid plate, the greater wing of the sphenoid is pierced by the large foramen ovale and the small foramen spinosum. Posterolateral to the foramen spinosum is the spine of the sphenoid. Behind the spine of the sphenoid, in the interval between the greater wing of the sphenoid and the petrous part of the temporal bone, is a groove for the cartilaginous part of the auditory tube. The opening of the bony part of the tube can be identified. The mandibular fossa of the temporal bone and the articular tubercle form the upper articular surfaces for the temporomandibular joint. Separating the mandibular fossa from the tympanic plate posteriorly is the squamotympanic fissure, through the medial end of which the chorda tympani exits from the tympanic cavity. The styloid process of the temporal bone projects downward and forward from its inferior aspect. The opening of the carotid canal can be seen on the inferior surface of the petrous part of the temporal bone. The medial end of the petrous part of the temporal bone is irregular and, together with the basilar part of the occipital bone and the greater wing of the sphenoid, forms the foramen lacerum. During life, the foramen lacerum is closed with fibrous tissue, and only a few small vessels pass through this foramen from the cavity of the skull to the exterior. The tympanic plate, which forms part of the temporal bone, is C shaped on section and forms the bony part of the external auditory meatus. While examining this region, identify the suprameatal crest on the lateral surface of the squamous part of the temporal bone, the suprameatal triangle, and the suprameatal spine. In the interval between the styloid and mastoid processes, the stylomastoid foramen can be seen. Medial to the styloid process, the petrous part of the temporal bone has a deep notch, which, together with a shallower notch on the occipital bone, forms the jugular foramen. Behind the posterior apertures of the nose and in front of the foramen magnum are the sphenoid bone and the basilar part of the occipital bone. The occipital condyles should be identified; they articulate with the superior aspect of the lateral mass of the first cervical vertebra, the atlas. Superior to the occipital condyle is the hypoglossal canal for transmission of the hypoglossal nerve (Fig. 5-6). Posterior to the foramen magnum in the midline is the external occipital protuberance. Neonatal Skull The newborn skull (Fig. 5-6), compared with the adult skull, has a disproportionately large cranium relative to the face. In childhood, the growth of the mandible, the maxillary sinuses, and the alveolar processes of the maxillae results in a great increase in length of the face. The bones of the skull are smooth and unilaminar, there being no diploë present. Most of the skull bones are ossified at birth, but the process is incomplete, and the bones are mobile on each other, being connected by fibrous tissue or cartilage. The bones of the vault are not closely knit at sutures, as in the adult, but are separated by unossified membranous intervals called fontanelles. Clinically, the anterior and posterior fontanelles are most important and are easily examined in the midline of the vault. The anterior fontanelle is diamond shaped and lies between the two halves of the frontal bone in front and the two parietal bones behind (Fig. 5-6). The fibrous membrane forming the floor of the anterior fontanelle is replaced by bone and is closed by 18 months of age. The posterior fontanelle P.191 P.192 is triangular and lies between the two parietal bones in front and the occipital bone behind. By the end of the first year, the fontanelle is usually closed and can no longer be palpated.

Figure 5-4 Bones of the skull viewed from the posterior (A) and superior (B) aspects.
Figure 5-5 Inferior surface of the base of the skull.

The tympanic part of the temporal bone is merely a C-shaped ring at birth, compared with a C-shaped curved plate in the adult. The mastoid process is not present at birth (Fig. 5-7) and develops later in response to the pull of the sternocleidomastoid muscle when the child moves his or her head. The mandible has right and left halves at birth, united in the midline with fibrous tissue. The two halves fuse at the symphysis menti by the end of the first year. The Cranial Cavity The cranial cavity contains the brain and its surrounding meninges, portions of the cranial nerves, arteries, veins, and venous sinuses. Vault of the Skull The internal surface of the vault shows the coronal, sagittal, and lambdoid sutures. In the midline is a shallow sagittal groove that lodges the superior sagittal sinus. Several narrow grooves are present for the anterior and posterior P.193 divisions of the middle meningeal vessels as they pass up the side of the skull to the vault.

Figure 5-6 Internal surface of the base of the skull.

Base of the Skull The interior of the base of the skull (Fig. 5-6) is divided into three cranial fossae: anterior, middle, and posterior. The anterior cranial fossa is separated from the middle cranial fossa by the lesser wing of the sphenoid, and the middle cranial fossa is separated from the posterior cranial fossa by the petrous part of the temporal bone. Anterior Cranial Fossa The anterior cranial fossa lodges the frontal lobes of the cerebral hemispheres. It is bounded anteriorly by the inner surface of the frontal bone, and in the midline is a crest for the attachment of the falx cerebri. Its posterior boundary is the sharp lesser wing of the sphenoid, which articulates laterally with the frontal bone and meets the anteroinferior angle of the parietal bone, or the pterion. The medial end of the lesser wing of the sphenoid forms the anterior clinoid process on each side, which gives attachment to the tentorium cerebelli. The median part of the anterior cranial fossa is limited posteriorly by the groove for the optic chiasma. The floor of the fossa is formed by the ridged orbital plates of the frontal bone laterally and by the cribriform plate of the ethmoid medially (Fig. 5-6). The crista galli is a sharp upward projection of the ethmoid bone in the midline for the attachment of the falx cerebri. Between the crista galli and the crest of the frontal bone is a small aperture, the foramen cecum, for the transmission of a small vein from the nasal mucosa to the superior sagittal sinus. P.194 Alongside the crista galli is a narrow slit in the cribriform plate for the passage of the anterior ethmoidal nerve into the nasal cavity. The upper surface of the cribriform plate supports the olfactory bulbs, and the small perforations in the cribriform plate are for the olfactory nerves.

Figure 5-7 Neonatal skull as seen from the anterior (A) and lateral (B) aspects.

Middle Cranial Fossa The middle cranial fossa consists of a small median part and expanded lateral parts (Fig. 5-6). The median raised part is formed by the body of the sphenoid, and the expanded lateral parts form concavities on either side, which lodge the temporal lobes of the cerebral hemispheres. It is bounded anteriorly by the sharp posterior edges of the lesser wings of the sphenoid and posteriorly by the superior borders of the petrous parts of the temporal bones. Laterally lie the squamous parts of the temporal bones, the greater wings of the sphenoid, and the parietal bones. The floor of each lateral part of the middle cranial fossa is formed by the greater wing of the sphenoid and the squamous and petrous parts of the temporal bone. The sphenoid bone resembles a bat having a centrally placed body with greater and lesser wings that are outstretched on each side. The body of the sphenoid contains the sphenoid air sinuses, which are lined with mucous membrane and communicate with the nasal cavity; they serve as voice resonators. Anteriorly, the optic canal transmits the optic nerve and the ophthalmic artery, a branch of the internal carotid artery, to the orbit. The superior orbital fissure, which is a slitlike opening between the lesser and greater wings of the sphenoid, transmits the lacrimal, frontal, trochlear, oculomotor, nasociliary, and abducent nerves, together with the superior ophthalmic vein. The sphenoparietal venous sinus runs medially along the posterior border of the lesser wing of the sphenoid and drains into the cavernous sinus. The foramen rotundum, which is situated behind the medial end of the superior orbital fissure, perforates the greater P.195 wing of the sphenoid and transmits the maxillary nerve from the trigeminal ganglion to the pterygopalatine fossa. The foramen ovale lies posterolateral to the foramen rotundum (Fig. 5-6). It perforates the greater wing of the sphenoid and transmits the large sensory root and small motor root of the mandibular nerve to the infratemporal fossa. The small foramen spinosum lies posterolateral to the foramen ovale and also perforates the greater wing of the sphenoid. The foramen transmits the middle meningeal artery from the infratemporal fossa into the cranial cavity. The artery then runs forward and laterally in a groove on the upper surface of the squamous part of the temporal bone and the greater wing of the sphenoid see Fig. 5-6). After a short distance, the artery divides into anterior and posterior branches. The anterior branch passes forward and upward to the anteroinferior angle of the parietal bone (see Fig. 15-5). Here, the bone is deeply grooved or tunneled by the artery for a short distance before it runs backward and upward on the parietal bone. It is at this site that the artery may be damaged after a blow to the side of the head. The posterior branch passes backward and upward across the squamous part of the temporal bone to reach the parietal bone. The large and irregularly shaped foramen lacerum lies between the apex of the petrous part of the temporal bone and the sphenoid bone (Fig. 5-6). The inferior opening of the foramen lacerum in life is filled by cartilage and fibrous tissue, and only small blood vessels pass through this tissue from the cranial cavity to the neck. The carotid canal opens into the side of the foramen lacerum above the closed inferior opening. The internal carotid artery enters the foramen through the carotid canal and immediately turns upward to reach the side of the body of the sphenoid bone. Here, the artery turns forward in the cavernous sinus to reach the region of the anterior clinoid process. At this point, the internal carotid artery turns vertically upward, medial to the anterior clinoid process, and emerges from the cavernous sinus (see p. 475). Lateral to the foramen lacerum is an impression on the apex of the petrous part of the temporal bone for the trigeminal ganglion. On the anterior surface of the petrous bone are two grooves for nerves; the largest medial groove is for the greater petrosal nerve, a branch of the facial nerve; the smaller lateral groove is for the lesser petrosal nerve, a branch of the tympanic plexus. The greater petrosal nerve enters the foramen lacerum deep to the trigeminal ganglion and joins the deep petrosal nerve (sympathetic fibers from around the internal carotid artery), to form the nerve of the pterygoid canal. The lesser petrosal nerve passes forward to the foramen ovale. The abducent nerve bends sharply forward across the apex of the petrous bone, medial to the trigeminal ganglion. Here, it leaves the posterior cranial fossa and enters the cavernous sinus. The arcuate eminence is a rounded eminence found on the anterior surface of the petrous bone and is caused by the underlying superior semicircular canal. The tegmen tympani, a thin plate of bone, is a forward extension of the petrous part of the temporal bone and adjoins the squamous part of the bone (Fig. 5-6). From behind forward, it forms the roof of the mastoid antrum, the tympanic cavity, and the auditory tube. This thin plate of bone is the only major barrier that separates infection in the tympanic cavity from the temporal lobe of the cerebral hemisphere. The median part of the middle cranial fossa is formed by the body of the sphenoid bone (Fig. 5-6). In front is the sulcus chiasmatis, which is related to the optic chiasma and leads laterally to the optic canal on each side. Posterior to the sulcus is an elevation, the tuberculum sellae. Behind the elevation is a deep depression, the sella turcica, which lodges the hypophysis cerebri. The sella turcica is bounded posteriorly by a square plate of bone called the dorsum sellae. The superior angles of the dorsum sellae have two tubercles, called the posterior clinoid processes, which give attachment to the fixed margin of the tentorium cerebelli. The cavernous sinus is directly related to the side of the body of the sphenoid (see p. 434). It carries in its lateral wall the third and fourth cranial nerves and the ophthalmic and maxillary divisions of the fifth cranial nerve (see Fig. 15-6). The internal carotid artery and the sixth cranial nerve pass forward through the sinus. Posterior Cranial Fossa The posterior cranial fossa is deep and lodges the parts of the hindbrain, namely, the cerebellum, pons, and medulla oblongata. Anteriorly, the fossa is bounded by the superior border of the petrous part of the temporal bone; posteriorly, it is bounded by the internal surface of the squamous part of the occipital bone (Fig. 5-6). The floor of the posterior fossa is formed by the basilar, condylar, and squamous parts of the occipital bone and the mastoid part of the temporal bone. The roof of the fossa is formed by a fold of dura, the tentorium cerebelli, which intervenes between the cerebellum below and the occipital lobes of the cerebral hemispheres above (see Fig. 15-3). The foramen magnum occupies the central area of the floor and transmits the medulla oblongata and its surrounding meninges, the ascending spinal parts of the accessory nerves, and the two vertebral arteries. The hypoglossal canal is situated above the anterolateral boundary of the foramen magnum (Fig. 5-6) and transmits the hypoglossal nerve. The jugular foramen lies between the lower border of the petrous part of the temporal bone and the condylar part of the occipital bone. It transmits the following structures from before backward: the inferior petrosal sinus; the 9th, 10th, and 11th cranial nerves; and the large sigmoid sinus. The inferior petrosal sinus descends in the groove on the lower border of the petrous part of the temporal bone to reach the foramen. The sigmoid sinus turns down through the foramen to become the internal jugular vein. P.196 The internal acoustic meatus pierces the posterior surface of the petrous part of the temporal bone. It transmits the vestibulocochlear nerve and the motor and sensory roots of the facial nerve. The internal occipital crest runs upward in the midline posteriorly from the foramen magnum to the internal occipital protuberance; to it is attached the small falx cerebelli over the occipital sinus. On each side of the internal occipital protuberance is a wide groove for the transverse sinus (Fig. 5-6). This groove sweeps around on either side, on the internal surface of the occipital bone, to reach the posteroinferior angle or corner of the parietal bone. The groove now passes onto the mastoid part of the temporal bone; at this point, the transverse sinus becomes the sigmoid sinus. The superior petrosal sinus runs backward along the upper border of the petrous bone in a narrow groove and drains into the sigmoid sinus. As the sigmoid sinus descends to the jugular foramen, it deeply grooves the back of the petrous bone and the mastoid part of the temporal bone. Here, it lies directly posterior to the mastoid antrum. Table 5-1 provides a summary of the more important openings in the base of the skull and the structures that pass through them.

Table 5-1 Summary of the More Important Openings in the Base of the Skull and the Structures That Pass Through Them
Opening in Skull Bone of Skull Structures Transmitted
Anterior Cranial Fossa
Perforations in cribriform plate Ethmoid Olfactory nerves
Middle Cranial Fossa
Optic canal Lesser wing of sphenoid Optic nerve, opthalmic artery
Superior orbital fissure Between lesser and greater wings of sphenoid Lacrimal, frontal, trochlear oculomotor, nasociliary, and abducent nerves; superior ophthalmic vein
Foramen rotundum Greater wing of sphenoid Maxillary division of the trigeminal nerve
Foramen ovale Greater wing of sphenoid Mandibular division of the trigeminal nerve, lesser petrosal nerve
Foramen spinosum Greater wing of sphenoid Middle meningeal artery
Foramen lacerum Between petrous part of temporal and sphenoid Internal carotid artery
Posterior Cranial Fossa
Foramen magnum Occipital Medulla oblongata, spinal part of accessory nerve, and right and left vertebral arteries
Hypoglossal canal Occipital Hypoglossal nerve
Jugular foramen Between petrous part of temporal and condylar part of occipital Glossopharyngeal, vagus, and accessory nerves; sigmoid sinus becomes internal jugular vein
Internal acoustic meatus Petrous part of temporal Vestibulocochlear and facial nerves

Mandible The mandible, or lower jaw, is the largest and strongest bone of the face, and it articulates with the skull at the temporamandibular joint (Fig. 5-3). The mandible consists of a horseshoe-shaped body and a pair of rami (Fig. 5-1). The body of the mandible meets the ramus on each side at the angle of the mandible. Introduction to the Brainstem The brainstem is made up of the medulla oblongata, the pons, and the midbrain and occupies the posterior cranial fossa of the skull (Fig. 5-8). It is stalklike in shape and connects the narrow spinal cord with the expanded forebrain (see Atlas Plates 1, 2, 3, 4, 5, 6, 7, 8). The brainstem has three broad functions: (1) it serves as a conduit for the ascending tracts and descending tracts connecting the spinal cord to the different parts of the higher centers in the forebrain; (2) it contains important reflex centers associated with the control of respiration and P.197the cardiovascular system and with the control of consciousness; and (3) it contains the important nuclei of cranial nerves III through XII.

Figure 5-8 Posterior view of the brainstem after removal of the occipital and parietal bones and the cerebrum, the cerebellum, and the roof of the fourth ventricle. Laminae of the upper cervical vertebrae have also been removed.

Gross Appearance of the Medulla Oblongata The medulla oblongata connects the pons superiorly with the spinal cord inferiorly (Fig. 5-8). The junction of the medulla and spinal cord is at the origin of the anterior and posterior roots of the first cervical spinal nerve, which corresponds approximately to the level of the foramen magnum. The medulla oblongata is conical in shape, its broad extremity being directed superiorly (Fig. 5-9). The central canal of the spinal cord continues upward into the lower half of the medulla; in the upper half of the medulla, it expands as the cavity of the fourth ventricle (Fig. 5-9). On the anterior surface of the medulla is the anterior median fissure, which is continuous inferiorly with the anterior median fissure of the spinal cord (Fig. 5-9). On each side of the median fissure, there is a swelling called the pyramid. The pyramids are composed of bundles of nerve fibers, called corticospinal fibers, which originate in large nerve cells in the precentral gyrus of the cerebral cortex. The pyramids taper inferiorly, and it is here that the majority of the descending fibers cross over to the opposite side, forming the decussation of the pyramids (Fig. 5-9). The anterior external arcuate fibers are a few nerve fibers that emerge from the anterior median fissure above the P.198 decussation and pass laterally over the surface of the medulla oblongata to enter the cerebellum. Posterolateral to the pyramids are the olives, which are oval elevations produced by the underlying inferior olivary nuclei. In the groove between the pyramid and the olive emerge the rootlets of the hypoglossal nerve. Posterior to the olives are the inferior cerebellar peduncles (Fig. 5-9), which connect the medulla to the cerebellum. In the groove between the olive and the inferior cerebellar peduncle emerge the roots of the glossopharyngeal and vagus nerves and the cranial roots of the accessory nerve (Fig. 5-9).

Figure 5-9 Medulla oblongata. A: Anterior view. B: Posterior view. Note that the roof of the fourth ventricle and the cerebellum have been removed.

The posterior surface of the superior half of the medulla oblongata forms the lower part of the floor of the fourth ventricle (Fig. 5-9). The posterior surface of the inferior half of the medulla is continuous with the posterior aspect of the spinal cord and possesses a posterior median sulcus. On each side of the median sulcus, there is an elongated swelling, the gracile tubercle, produced by the underlying gracile nucleus (Fig. 5-9). Lateral to the gracile tubercle is a similar swelling, the cuneate tubercle, produced by the underlying cuneate nucleus. Internal Structure As in the spinal cord, the medulla oblongata consists of white matter and gray matter, but a study of transverse sections of this region shows that they have been extensively rearranged. This rearrangement can be explained embryologically by the P.199 expansion of the neural tube to form the hindbrain vesicle, which becomes the fourth ventricle (Fig. 5-10). The extensive lateral spread of the fourth ventricle results in an alteration in the position of the derivatives of the alar and basal plates of the embryo. To assist in understanding this concept, remember that in the spinal cord, the derivatives of the alar and basal plates are situated posterior and anterior to the sulcus limitans, respectively, and in the case of the medulla oblongata, they are situated lateral and medial to the sulcus limitans, respectively (Fig. 5-10). The internal structure of the medulla oblongata is considered at four levels: (1) level of decussation of pyramids, (2) level of decussation of lemnisci, (3) level of the olives, and (4) level just inferior to the pons. See Table 5-2 for a comparison of the different levels of the medulla oblongata and the major structures present at each level.

Figure 5-10 Stages in the development of the spinal cord (A–D) and the medulla oblongata (E, F). The neural crest cells will form the first afferent sensory neurons in the posterior root ganglia of the spinal nerves and the sensory ganglia of the cranial nerves.

Level of Decussation of Pyramids A transverse section through the inferior half of the medulla oblongata (Figs. 5-11A and 5-12) passes through the decussation of the pyramids, the great motor decussation. In the superior part of the medulla, the corticospinal fibers occupy and form the pyramid, but inferiorly, about three-fourths of the fibers cross the median plane and continue down the spinal cord in the lateral white column P.200 as the lateral corticospinal tract. As these fibers cross the midline, they sever the continuity between the anterior column of the gray matter of the spinal cord and the gray matter that surrounds the central canal.

Table 5-2 Comparison of the Different Levels of the Medulla Oblongata Showing the Major Structures at Each Levela
Level Cavity Nuclei Motor Tracts Sensory Tracts
Decussation of pyramids Central canal Nucleus gracilis, nucleus cuneatus, spinal nucleus of cranial nerve V, accessory nucleus Decussation of corticospinal tracts, pyramids Spinal tract of cranial nerve V, posterior spinocerebellar tract, lateral spinothalamic tract, anterior spinocerebellar tract
Decussation of medial lemnisci Central canal Nucleus gracilis, nucleus cuneatus, spinal nucleus of cranial nerve V, accessory nucleus, hypoglossal nucleus Pyramids Decussation of medial lemnisci, fasciculus gracilis, fasciculus cuneatus, spinal tract of cranial nerve V, posterior spinocerebellar tract, lateral spinothalamic tract, anterior spinocerebellar tract
Olives, inferior cerebellar peduncle Fourth ventricle Inferior olivary nucleus, spinal nucleus of cranial nerve V, vestibular nucleus, glossopharyngeal nucleus, vagal nucleus, hypoglossal nucleus, nucleus ambiguus, nucleus of tractus solitarius Pyramids Medial longitudinal fasciculus, tectospinal tract, medial lemniscus, spinal tract of cranial nerve V, lateral spinothalamic tract, anterior spinocerebellar tract
Just inferior to pons Fourth ventricle Lateral vestibular nucleus, cochlear nuclei   No major changes in distribution of gray and white matter
aNote that the reticular formation is present at all levels.

The fasciculus gracilis and the fasciculus cuneatus continue to ascend superiorly posterior to the central gray matter (Figs. 5-11A and 5-12). The nucleus gracilis and the nucleus cuneatus appear as posterior extensions of the central gray matter. The substantia gelatinosa in the posterior gray column of the spinal cord becomes continuous with the inferior end of the nucleus of the spinal tract of the trigeminal nerve. The fibers of the tract of the nucleus are situated between the nucleus and the surface of the medulla oblongata. The lateral and anterior white columns of the spinal cord are easily identified in these sections, and their fiber arrangement is unchanged (Figs. 5-11A and 5-12). Level of Decussation of Lemnisci A transverse section through the inferior half of the medulla oblongata, a short distance above the level of the decussation of the pyramids, passes through the decussation of lemnisci, the great sensory decussation (Figs. 5-11B and 5-13). The decussation of the lemnisci takes place anterior to the central gray matter and posterior to the pyramids. It should be understood that the lemnisci have been formed from the internal arcuate fibers, which have emerged from the anterior aspects of the nucleus gracilis and nucleus cuneatus. The internal arcuate fibers first travel anteriorly and laterally around the central gray matter. They then curve medially toward the midline, where they decussate with the corresponding fibers of the opposite side (Figs. 5-11B and 5-13). The nucleus of the spinal tract of the trigeminal nerve lies lateral to the internal arcuate fibers. The spinal tract of the trigeminal nerve lies lateral to the nucleus (Figs. 5-11B and 5-13). The lateral and anterior spinothalamic tracts and the spinotectal tracts occupy an area lateral to the decussation of the lemnisci (Fig. 5-11B). They are very close to one another and collectively are known as the spinal lemniscus. The spinocerebellar, vestibulospinal, and the rubrospinal tracts are situated in the anterolateral region of the medulla oblongata. Level of the Olives A transverse section through the olives passes across the inferior part of the fourth ventricle (Figs. 5-14 and 5-15). P.201 P.202 P.203 P.204 The amount of gray matter has increased at this level owing to the presence of the olivary nuclear complex; the nuclei of the vestibulocochlear, glossopharyngeal, vagus, accessory, and hypoglossal nerves; and the arcuate nuclei.

Figure 5-11 Transverse sections of the medulla oblongata. A: Level of decussation of the pyramids. B: Level of decussation of the medial lemnisci.
Figure 5-12 Transverse section of the medulla oblongata at the level of decussation of the pyramids. (Weigert stain.)
Figure 5-13 Transverse section of the medulla oblongata at the level of decussation of the medial lemnisci. (Weigert stain.)
Figure 5-14 Transverse sections of the medulla oblongata at the level of the middle of the olivary nuclei (A) and the superior part of the olivary nuclei just inferior to the pons (B).
Figure 5-15 Transverse section of the medulla oblongata at the level of the middle of the olivary nuclei. (Weigert stain.)

Olivary Nuclear Complex The largest nucleus of this complex is the inferior olivary nucleus (Figs. 5-14 and 5-15). The gray matter is shaped like a crumpled bag with its mouth directed medially; it is responsible for the elevation on the surface of the medulla called the olive. Smaller dorsal and medial accessory olivary nuclei also are present. The cells of the inferior olivary nucleus send fibers medially across the midline to enter the cerebellum through the inferior cerebellar peduncle. Afferent fibers reach the inferior olivary nuclei from the spinal cord (the spino-olivary tracts) and from the cerebellum and cerebral cortex. The function of the olivary nuclei is associated with voluntary muscle movement. Vestibulocochlear Nuclei The vestibular nuclear complex is made up of the following nuclei: (1) medial vestibular nucleus, (2) inferior vestibular nucleus, (3) lateral vestibular nucleus, and (4) superior vestibular nucleus. The details of these nuclei and their connections are discussed later. The medial and inferior vestibular nuclei can be seen on section at this level (Figs. 5-14 and 5-15). There are two cochlear nuclei. The anterior cochlear nucleus is situated on the anterolateral aspect of the inferior cerebellar peduncle, and the posterior cochlear nucleus is situated on the posterior aspect of the peduncle lateral to the floor of the fourth ventricle (Figs. 5-14 and 5-15). The connections of these nuclei are described later (see page 350). The Nucleus Ambiguus The nucleus ambiguus consists of large motor neurons and is situated deep within the reticular formation (Figs. 5-14 and 5-16). The emerging nerve fibers join the glossopharyngeal, vagus, and cranial part of the accessory nerve and are distributed to voluntary skeletal muscle. Central Gray Matter The central gray matter lies beneath the floor of the fourth ventricle at this level (Figs. 5-14 and 5-15). Passing from medial to lateral (Fig. 5-16), the following important structures may be recognized: (1) the hypoglossal nucleus, (2) the dorsal nucleus of the vagus, (3) the nucleus of the tractus solitarius, and (4) the medial and inferior vestibular nuclei (see previous column). The nucleus ambiguus, referred to above, has become deeply placed within the reticular formation (Fig. 5-14). The connections and functional significance of these nuclei are described in Chapter 11. P.205

Figure 5-16 Position of the cranial nerve nuclei within the brainstem. The hatched area indicates the position of the vestibular nuclei.

The arcuate nuclei are thought to be inferiorly displaced pontine nuclei (see p. 208) and are situated on the anterior surface of the pyramids (Fig. 5-14). They receive nerve fibers from the cerebral cortex and send efferent fibers to the cerebellum through the anterior external arcuate fibers. The pyramids containing the corticospinal and some corticonuclear fibers are situated in the anterior part of the medulla separated by the anterior median fissure (Figs. 5-14 and 5-15); the corticospinal fibers descend to the spinal cord, and the corticonuclear fibers are distributed to the motor nuclei of the cranial nerves situated within the medulla. The medial lemniscus forms a flattened tract on each side of the midline posterior to the pyramid (Figs. 5-7 and 5-15). These fibers emerge from the decussation of the lemnisci and convey sensory information to the thalamus. The medial longitudinal fasciculus forms a small tract of nerve fibers situated on each side of the midline posterior to the medial lemniscus and anterior to the hypoglossal nucleus (Figs. 5-14 and 5-15). It consists of ascending and descending fibers, the connections of which are described on page 208. The inferior cerebellar peduncle is situated in the posterolateral corner of the section on the lateral side of the fourth ventricle (Figs. 5-14 and 5-15). The spinal tract of the trigeminal nerve and its nucleus are situated on the anteromedial aspect of the inferior cerebellar peduncle (Figs. 5-14 and 5-15). The anterior spinocerebellar tract is situated near the surface in the interval between the inferior olivary nucleus and the nucleus of the spinal tract of the trigeminal nerve (Figs. 5-14 and 5-15). The spinal lemniscus, consisting of the anterior spinothalamic, the lateral spinothalamic, and spinotectal tracts, is deeply placed. The reticular formation, consisting of a diffuse mixture of nerve fibers and small groups of nerve cells, is deeply placed posterior to the olivary nucleus (Figs. 5-14 and 5-15). The reticular formation represents, at this level, only a small part of this system, which is also present in the pons and midbrain. The glossopharyngeal, vagus, and cranial part of the accessory nerves can be seen running forward and laterally through the reticular formation (Fig. 5-14). The nerve fibers emerge between the olives and the inferior cerebellar peduncles. The hypoglossal nerves also run anteriorly and laterally through the reticular formation and emerge between the pyramids and the olives (Fig. 5-14). P.206 Level Just Inferior to the Pons There are no major changes, in comparison to the previous level, in the distribution of the gray and white matter (Figs. 5-14 and 5-16). The lateral vestibular nucleus has replaced the inferior vestibular nucleus, and the cochlear nuclei now are visible on the anterior and posterior surfaces of the inferior cerebellar peduncle. Gross Appearance of the Pons The pons is anterior to the cerebellum (Fig. 5-17; see also Fig. 6-1) and connects the medulla oblongata to the midbrain. It is about 1 inch (2.5 cm) long and owes its name to the appearance presented on the anterior surface, which is that of a bridge connecting the right and left cerebellar hemispheres. The anterior surface is convex from side to side and shows many transverse fibers that converge on each side to form the middle cerebellar peduncle (Fig. 5-17). There is a shallow groove in the midline, the basilar groove, which lodges the basilar artery. On the anterolateral surface of the pons, the trigeminal nerve emerges on each side. Each nerve consists of a smaller, medial part, known as the motor root, and a larger, lateral part, known as the sensory root. In the groove between the pons and the medulla oblongata, there emerge, from medial to lateral, the abducent, facial, and vestibulocochlear nerves (Fig. 5-17).

Figure 5-17 Anterior surface of the brainstem showing the pons.

The posterior surface of the pons is hidden from view by the cerebellum (Fig. 5-18). It forms the upper half of the floor of the fourth ventricle and is triangular in shape. The posterior surface is limited laterally by the superior cerebellar peduncles and is divided into symmetrical halves by a median sulcus. Lateral to this sulcus is an elongated elevation, the medial eminence, which is bounded laterally by a sulcus, the sulcus limitans (Fig. 5-18). The inferior end of the medial eminence is slightly expanded to form the facial colliculus, which is produced by the root of the facial nerve winding around the nucleus of the abducent nerve (Fig. 5-19). The floor of the superior part of the sulcus limitans is bluish-gray in color and is called the substantia ferruginea; it owes its color to a group of deeply pigmented nerve cells. Lateral to the sulcus limitans is the area vestibuli produced by the underlying vestibular nuclei (Fig. 5-18). P.207

Figure 5-18 Posterior surface of the brainstem showing the pons. The cerebellum has been removed.
Figure 5-19 Transverse section through the caudal part of the pons at the level of the facial colliculus.

P.208 Internal Structure of the Pons For purposes of description, the pons is commonly divided into a posterior part, the tegmentum, and an anterior basal part by the transversely running fibers of the trapezoid body (Fig. 5-19). The structure of the pons may be studied at two levels: (1) transverse section through the caudal part, passing through the facial colliculus, and (2) transverse section through the cranial part, passing through the trigeminal nuclei. See Table 5-3 for a comparison of the two levels of the pons and the major structures present at each level. Transverse Section Through the Caudal Part The medial lemniscus rotates as it passes from the medulla into the pons. It is situated in the most anterior part of the tegmentum, with its long axis running transversely (Fig. 5-19). The medial lemniscus is accompanied by the spinal and lateral lemnisci. The facial nucleus lies posterior to the lateral part of the medial lemniscus. The fibers of the facial nerve wind around the nucleus of the abducent nerve, producing the facial colliculus (Fig. 5-19). The fibers of the facial nerve then pass anteriorly between the facial nucleus and the superior end of the nucleus of the spinal tract of the trigeminal nerve. The medial longitudinal fasciculus is situated beneath the floor of the fourth ventricle on either side of the midline (Fig. 5-19). The medial longitudinal fasciculus is the main pathway that connects the vestibular and cochlear nuclei with the nuclei controlling the extraocular muscles (oculomotor, trochlear, and abducent nuclei). The medial vestibular nucleus is situated lateral to the abducent nucleus (Fig. 5-19) and is in close relationship to the inferior cerebellar peduncle. The superior part of the lateral and the inferior part of the superior vestibular nucleus are found at this level. The posterior and anterior cochlear nuclei are also found at this level.

Table 5-3 Comparison of the Different Levels of the Pons Showing the Major Structures at Each Levela
Level Cavity Nuclei Motor Tracts Sensory Tracts
Facial colliculus Fourth ventricle Facial nucleus, abducent nucleus, medial vestibular nucleus, spinal nucleus of cranial nerve V, pontine nuclei, trapezoid nuclei Corticospinal and corticonuclear tracts, transverse pontine fibers, medial longitudinal fasciculus Spinal tract of cranial nerve V; lateral, spinal, and medial lemnisci
Trigeminal nuclei Fourth ventricle Main sensory and motor nucleus of cranial nerve V, pontine nuclei, trapezoid nuclei Corticospinal and corticonuclear tracts, transverse pontine fibers, medial longitudinal fasciculus Lateral, spinal, and medial lemnisci
aNote that the reticular formation is present at all levels.

The spinal nucleus of the trigeminal nerve and its tract lie on the anteromedial aspect of the inferior cerebellar peduncle (Fig. 5-19). The trapezoid body is made up of fibers derived from the cochlear nuclei and the nuclei of the trapezoid body. They run transversely (Fig. 5-19) in the anterior part of the tegmentum (see p. 207). The basilar part of the pons, at this level, contains small masses of nerve cells called pontine nuclei (Fig. 5-19). The corticopontine fibers of the crus cerebri of the midbrain terminate in the pontine nuclei. The axons of these cells give origin to the transverse fibers of the pons, which cross the midline and intersect the corticospinal and corticonuclear tracts, breaking them up into small bundles. The transverse fibers of the pons enter the middle cerebellar peduncle and are distributed to the cerebellar hemisphere. This connection forms the main pathway linking the cerebral cortex to the cerebellum. Transverse Section Through the Cranial Part The internal structure of the cranial part of the pons is similar to that seen at the caudal level (Figs. 5-20, 5-21 and 5-22), but it now contains the motor and principal sensory nuclei of the trigeminal nerve. The motor nucleus of the trigeminal nerve is situated beneath the lateral part of the fourth ventricle within the reticular formation (Figs. 5-20 and 5-21). The emerging motor fibers travel anteriorly through the substance of the pons and exit on its anterior surface. The principal sensory nucleus of the trigeminal nerve is situated on the lateral side of the motor nucleus (Figs. 5-20 and 5-21); it is continuous inferiorly with the nucleus of the spinal tract. The entering sensory fibers travel through the substance of the pons and lie lateral to the motor fibers (Fig. 5-20). P.209

Figure 5-20 Transverse section through the pons at the level of the trigeminal nuclei.
Figure 5-21 Photomicrograph of a transverse section of the pons at the level of the trigeminal nuclei.

P.210

Figure 5-22 Photomicrograph of a transverse section of the most rostral part of the pons.

The superior cerebellar peduncle is situated posterolateral to the motor nucleus of the trigeminal nerve (Figs. 5-20 and 5-21). It is joined by the anterior spinocerebellar tract. The trapezoid body and the medial lemniscus are situated in the same position as they were in the previous section (Fig. 5-20). The lateral and spinal lemnisci lie at the lateral extremity of the medial lemniscus (Figs. 5-20 and 5-22). Gross Appearance of the Midbrain The midbrain measures about 0.8 inch (2 cm) in length and connects the pons and cerebellum with the forebrain (Fig. 5-23). Its long axis inclines anteriorly as it ascends through the opening in the tentorium cerebelli. The midbrain is traversed by a narrow channel, the cerebral aqueduct, which is filled with cerebrospinal fluid (Figs. 5-24, 5-25, 5-26, 5-27 and 5-28). On the posterior surface are four colliculi (corpora quadrigemina). These are rounded eminences that are divided into superior and inferior pairs by a vertical and a transverse groove (Fig. 5-26). The superior colliculi are centers for visual reflexes (see p. 338), and the inferior colliculi are lower auditory centers. In the midline below the inferior colliculi, the trochlear nerves emerge. These are small-diameter nerves that wind around the lateral aspect of the midbrain to enter the lateral wall of the cavernous sinus. On the lateral aspect of the midbrain, the superior and inferior brachia ascend in an anterolateral direction (Fig. 5-23). The superior brachium passes from the superior colliculus to the lateral geniculate body and the optic tract. The inferior brachium connects the inferior colliculus to the medial geniculate body. On the anterior aspect of the midbrain (Fig. 5-23), there is a deep depression in the midline, the interpeduncular fossa, which is bounded on either side by the crus cerebri. Many small blood vessels perforate the floor of the interpeduncular fossa, and this region is termed the posterior perforated substance (Fig. 5-23). The oculomotor nerve emerges from a groove on the medial side of the crus cerebri and passes forward in the lateral wall of the cavernous sinus. Internal Structure of the Midbrain The midbrain comprises two lateral halves, called the cerebral peduncles; each of these is divided into an anterior part, the crus cerebri, and a posterior part, the tegmentum, by a pigmented band of gray matter, the substantia nigra (Figs. 5-24 and 5-25). The narrow cavity of the P.211 midbrain is the cerebral aqueduct, which connects the third and fourth ventricles. The tectum is the part of the midbrain posterior to the cerebral aqueduct; it has four small surface swellings referred to previously; these are the two superior and two inferior colliculi (Figs. 5-24 and 5-25). The cerebral aqueduct is lined by ependyma and is surrounded by the central gray matter. On transverse sections of the midbrain, the interpeduncular fossa can be seen to separate the crura cerebri, whereas the tegmentum is continuous across the median plane (Fig. 5-24).

Figure 5-23 The midbrain. A: Anterior view. B: Lateral view.

Transverse Section of the Midbrain at the Level of the Inferior Colliculi The inferior colliculus, consisting of a large nucleus of gray matter, lies beneath the corresponding surface elevation and forms part of the auditory pathway (Figs. 5-25A and 5-27). It receives many of the terminal fibers of the lateral lemniscus. The pathway then continues through the inferior brachium to the medial geniculate body. P.212

Figure 5-24 Transverse section of the midbrain through the inferior colliculi shows the division of the midbrain into the tectum and the cerebral peduncles. Note that the cerebral peduncles are subdivided by the substantia nigra into the tegmentum and the crus cerebri.

The trochlear nucleus is situated in the central gray matter close to the median plane just posterior to the medial longitudinal fasciculus. The emerging fibers of the trochlear nucleus pass laterally and posteriorly around the central gray matter and leave the midbrain just below the inferior colliculi. The fibers of the trochlear nerve now decussate completely in the superior medullary velum. The mesencephalic nuclei of the trigeminal nerve are lateral to the cerebral aqueduct (Figs. 5-25A and 5-27). The decussation of the superior cerebellar peduncles occupies the central part of the tegmentum anterior to the cerebral aqueduct. The reticular formation is smaller than that of the pons and is situated lateral to the decussation. The medial lemniscus ascends posterior to the substantia nigra; the spinal and trigeminal lemnisci are situated lateral to the medial lemniscus (Figs. 5-25 and 5-27). The lateral lemniscus is located posterior to the trigeminal lemniscus. The substantia nigra (Figs. 5-25 and 5-27) is a large motor nucleus situated between the tegmentum, and the crus cerebri and is found throughout the midbrain. The nucleus is composed of medium-size multipolar neurons that possess inclusion granules of melanin pigment within their cytoplasm. The substantia nigra is concerned with muscle tone and is connected to the cerebral cortex, spinal cord, hypothalamus, and basal nuclei. The crus cerebri contains important descending tracts and is separated from the tegmentum by the substantia nigra (Figs. 5-25 and 5-27). The corticospinal and corticonuclear fibers occupy the middle two-thirds of the crus. The frontopontine fibers occupy the medial part of the crus, and the temporopontine fibers occupy the lateral part of the crus (Figs. 5-25 and 5-27). These descending tracts connect the cerebral cortex to the anterior gray column cells of the spinal cord, the cranial nerve nuclei, the pons, and the cerebellum (Table 5-4). Transverse Section of the Midbrain at the Level of the Superior Colliculi The superior colliculus (Figs. 5-25B and 5-28), a large nucleus of gray matter that lies beneath the corresponding surface elevation, forms part of the visual reflexes. It is connected to the lateral geniculate body by the superior brachium. It receives afferent fibers from the optic nerve, the P.213 P.214 P.215visual cortex, and the spinotectal tract. The efferent fibers form the tectospinal and tectobulbar tracts, which are probably responsible for the reflex movements of the eyes, head, and neck in response to visual stimuli. The afferent pathway for the light reflex ends in the pretectal nucleus. This is a small group of neurons situated close to the lateral part of the superior colliculus. After relaying in the pretectal nucleus, the fibers pass to the parasympathetic nucleus of the oculomotor nerve (Edinger-Westphal nucleus). The emerging fibers then pass to the oculomotor nerve. The oculomotor nucleus is situated in the central gray matter close to the median plane, just posterior to the medial longitudinal fasciculus (Figs. 5-25B and 5-28). The fibers of the oculomotor nucleus pass anteriorly through the red nucleus to emerge on the medial side of the crus cerebri in the interpeduncular fossa. The nucleus of the oculomotor nerve is divisible into a number of cell groups.

Figure 5-25 Transverse sections of the midbrain. A: At the level of the inferior colliculus. B: At the level of the superior colliculus. Note that trochlear nerves completely decussate within the superior medullary velum.
Figure 5-26 Posterior view of the brainstem showing the two superior and the two inferior colliculi of the tectum.
Figure 5-27 Photomicrograph of a transverse section of the midbrain at the level of the inferior colliculus. (Weigert stain.)
Figure 5-28 Photomicrograph of a transverse section of the midbrain at the level of the superior colliculus. (Weigert stain.)
Table 5-4 Comparison of Two Levels of the Midbrain Showing the Major Structures at Each Levela
Level Cavity Nuclei Motor Tract Sensory Tracts
Inferior colliculi Cerebral aqueduct Inferior colliculus, substantia nigra, trochlear nucleus, mesencephalic nuclei of cranial nerve V Corticospinal and corticonuclear tracts, temporopontine, frontopontine, medial longitudinal fasciculus Lateral, trigeminal, spinal, and medial lemnisci; decussation of superior cerebellar peduncles
Superior colliculi Cerebral aqueduct Superior colliculus, substantia nigra, oculomotor nucleus, Edinger-Westphal nucleus, red nucleus, mesencephalic nucleus of cranial nerve V Corticospinal and corticonuclear tracts, temporopontine, frontopontine, medial longitudinal fasciculus, decussation of rubrospinal tract Trigeminal, spinal, and medial lemnisci
aNote that the reticular formation is present at all levels.

The medial, spinal, and trigeminal lemnisci form a curved band posterior to the substantia nigra, but the lateral lemniscus does not extend superiorly to this level (Figs. 5-25B and 5-28). The red nucleus (Figs. 5-25B and 5-28) is a rounded mass of gray matter situated between the cerebral aqueduct and the substantia nigra. Its reddish hue, seen in fresh specimens, is due to its vascularity and the presence of an iron-containing pigment in the cytoplasm of many of its neurons. P.216Afferent fibers reach the red nucleus from (1) the cerebral cortex through the corticospinal fibers, (2) the cerebellum through the superior cerebellar peduncle, and (3) the lentiform nucleus, subthalamic and hypothalamic nuclei, substantia nigra, and spinal cord. Efferent fibers leave the red nucleus and pass to (1) the spinal cord through the rubrospinal tract (as this tract descends, it decussates), (2) the reticular formation through the rubroreticular tract, (3) the thalamus, and (4) the substantia nigra.

Figure 5-29 Position of some of the cranial nerve nuclei in the brainstem. A: Surface projection on the posterior aspect of the brainstem. B: Cross sections. The motor nuclei are in red and the sensory nuclei in blue.

The reticular formation is situated in the tegmentum lateral and posterior to the red nucleus (Figs. 5-25B and 5-28). The crus cerebri contains the identical important descending tracts—the corticospinal, corticonuclear, and corticopontine fibers—that are present at the level of the inferior colliculus (see Table 5-4). The continuity of the various cranial nerve nuclei through the different regions of the brainstem is shown diagrammatically in Figure 5-29. P.217 P.218 P.219 P.220 P.221 Clinical Notes Clinical Significance of the Medulla Oblongata The medulla oblongata not only contains many cranial nerve nuclei that are concerned with vital functions (e.g., regulation of heart rate and respiration), but it also serves as a conduit for the passage of ascending and descending tracts connecting the spinal cord to the higher centers of the nervous system. These tracts may become involved in demyelinating diseases, neoplasms, and vascular disorders. Raised Pressure in the Posterior Cranial Fossa and Its Effect on the Medulla Oblongata The medulla oblongata is situated in the posterior cranial fossa, lying beneath the tentorium cerebelli and above the foramen magnum. It is related anteriorly to the basal portion of the occipital bone and the upper part of the odontoid process of the axis and posteriorly to the cerebellum. In patients with tumors of the posterior cranial fossa, the intracranial pressure is raised, and the brain—that is, the cerebellum and the medulla oblongata—tends to be pushed toward the area of least resistance; there is a downward herniation of the medulla and cerebellar tonsils through the foramen magnum. This will produce the symptoms of headache, neck stiffness, and paralysis of the glossopharyngeal, vagus, accessory, and hypoglossal nerves owing to traction. In these circumstances, it is extremely dangerous to perform a lumbar puncture because the sudden withdrawal of cerebrospinal fluid may precipitate further herniation of the brain through the foramen magnum and a sudden failure of vital functions, resulting from pressure and ischemia of the cranial nerve nuclei present in the medulla oblongata.

Figure 5-30 Arnold-Chiari phenomenon. This coronal section of the skull shows the herniation of the cerebellar tonsil and the medulla oblongata through the foramen magnum into the vertebral canal.

Arnold-Chiari Phenomenon The Arnold-Chiari malformation is a congenital anomaly in which there is a herniation of the tonsils of the cerebellum and the medulla oblongata through the foramen magnum into the vertebral canal (Fig. 5-30). This results in the blockage of the exits in the roof of the fourth ventricle to the cerebrospinal fluid, causing internal hydrocephalus. It is commonly associated with craniovertebral anomalies or various forms of spina bifida. Signs and symptoms related to pressure on the cerebellum and medulla oblongata and involvement of the last four cranial nerves are associated with this condition. Vascular Disorders of the Medulla Oblongata Lateral Medullary Syndrome of Wallenberg The lateral part of the medulla oblongata is supplied by the posterior inferior cerebellar artery, which is usually a branch of the vertebral artery. Thrombosis of either of these arteries (Fig. 5-31) produces the following signs and symptoms: dysphagia and dysarthria due to paralysis of the ipsilateral palatal and laryngeal muscles (innervated by the nucleus ambiguus); analgesia and thermoanesthesia on the ipsilateral side of the face (nucleus and spinal tract of the trigeminal nerve); vertigo, nausea, vomiting, and nystagmus (vestibular nuclei); ipsilateral Horner syndrome (descending sympathetic fibers); ipsilateral cerebellar signs—gait and limb ataxia (cerebellum or inferior cerebellar peduncle); and contralateral loss of sensations of pain and temperature (spinal lemniscus—spinothalamic tract).

Figure 5-31 Transverse section of the medulla oblongata at the level of the inferior olivary nuclei showing the extent of the lesion producing the lateral medullary syndrome.

Medial Medullary Syndrome The medial part of the medulla oblongata is supplied by the vertebral artery. Thrombosis of the medullary branch (Fig. 5-32) produces the following signs and symptoms: contralateral hemiparesis (pyramidal tract), contralateral impaired sensations of position and movement and tactile discrimination (medial lemniscus), and ipsilateral paralysis of tongue muscles with deviation to the paralyzed side when the tongue is protruded (hypoglossal nerve). Clinical Significance of the Pons The pons, like the medulla oblongata and the cerebellum, is situated in the posterior cranial fossa lying beneath the tentorium cerebelli. It is related anteriorly to the basilar artery, the dorsum sellae of the sphenoid bone, and the basilar part of the occipital bone. In addition to forming the upper half of the floor of the fourth ventricle, it possesses several important cranial nerve nuclei (trigeminal, abducent, facial, and vestibulocochlear) and serves as a conduit for important ascending and descending tracts (corticonuclear, corticopontine, corticospinal, medial longitudinal fasciculus and medial, spinal, and lateral lemnisci). It is not surprising, therefore, that tumors, hemorrhage, or infarcts in this area of the brain produce a variety of symptoms and signs. For example, involvement of the corticopontocerebellar tracts will produce marked cerebellar ataxia, and voluntary movements are accompanied by a rhythmic tremor that develops and becomes further accentuated as the movements proceed (intention tumor). Tumors of the Pons Astrocytoma of the pons occurring in childhood is the most common tumor of the brainstem. The symptoms and signs are those of ipsilateral cranial nerve paralysis and contralateral hemiparesis: weakness of the facial muscles on the same side (facial nerve nucleus), weakness of the lateral rectus muscle on one or both sides (abducent nerve nucleus), nystagmus (vestibular nucleus), weakness of the jaw muscles (trigeminal nerve nucleus), impairment of hearing (cochlear nuclei), contralateral hemiparesis, quadriparesis (corticospinal fibers), anesthesia to light touch with the preservation of appreciation of pain over the skin of the face (principal sensory nucleus of trigeminal nerve involved, leaving spinal nucleus and tract of trigeminal intact), and contralateral sensory defects of the trunk and limbs (medial and spinal lemnisci). Involvement of the corticopontocerebellar tracts may cause ipsilateral cerebellar signs and symptoms. There may be impairment of conjugate deviation of the eyeballs due to involvement of the medial longitudinal fasciculus, which connects the oculomotor, trochlear, and abducent nerve nuclei. Pontine Hemorrhage The pons is supplied by the basilar artery and the anterior, inferior, and superior cerebellar arteries. If the hemorrhage occurs from one of those arteries and is unilateral, there will be facial paralysis on the side of the lesion (involvement of the facial nerve nucleus and, therefore, a lower motor neuron palsy) and paralysis of the limbs on the opposite side (involvement of the corticospinal fibers as they pass through the pons). There is often paralysis of conjugate ocular deviation (involvement of the abducent nerve nucleus and the medial longitudinal fasciculus).

Figure 5-32 Transverse section of the medulla oblongata at the level of the inferior olivary nuclei showing the extent of the lesion producing the medial medullary syndrome.

When the hemorrhage is extensive and bilateral, the pupils may be “pinpoint” (involvement of the ocular sympathetic fibers); there is commonly bilateral paralysis of the face and the limbs. The patient may become poikilothermic because severe damage to the pons has cut off the body from the heat-regulating centers in the hypothalamus. Infarctions of the Pons Usually, infarction of the pons is due to thrombosis or embolism of the basilar artery or its branches. If it involves the paramedian area of the pons, the corticospinal tracts, the pontine nuclei, and the fibers passing to the cerebellum through the middle cerebellar peduncle may be damaged. A laterally situated infarct will involve the trigeminal nerve, the medial lemniscus, and the middle cerebellar peduncle; the corticospinal fibers to the lower limbs also may be affected. The clinical conditions mentioned above will be understood more clearly if the ascending and descending tracts of the brain and spinal cord are reviewed (see pp. 143 and 153). Clinical Significance of the Midbrain The midbrain forms the upper end of the narrow stalk of the brain or brainstem. As it ascends out of the posterior cranial fossa through the relatively small rigid opening in the tentorium cerebelli, it is vulnerable to traumatic injury. It possesses two important cranial nerve nuclei (oculomotor and trochlear), reflex centers (the colliculi), and the red nucleus and substantia nigra, which greatly influence motor function, and the midbrain serves as a conduit for many important ascending and descending tracts. As in other parts of the brainstem, it is a site for tumors, hemorrhage, or infarcts that will produce a wide variety of symptoms and signs. Trauma to the Midbrain Among the mechanisms of injuries to the midbrain, a sudden lateral movement of the head could result in the cerebral peduncles impinging against the sharp rigid free edge of the tentorium cerebelli. Sudden movements of the head resulting from trauma cause different regions of the brain to move at different velocities relative to one another. For example, the large anatomical unit, the forebrain, may move at a different velocity from the remainder of the brain, such as the cerebellum. This will result in the midbrain being bent, stretched, twisted, or torn. Involvement of the oculomotor nucleus will produce ipsilateral paralysis of the levator palpebrae superioris; the superior, inferior, and medial rectus muscles; and the inferior oblique muscle. Malfunction of the parasympathetic nucleus of the oculomotor nerve produces a dilated pupil that is insensitive to light and does not constrict on accommodation. Involvement of the trochlear nucleus will produce contralateral paralysis of the superior oblique muscle of the eyeball. Thus, it is seen that involvement of one or both of these nuclei, or the corticonuclear fibers that converge on them, will cause impairment of ocular movements. Blockage of the Cerebral Aqueduct The cavity of the midbrain, the cerebral aqueduct, is one of the narrower parts of the ventricular system. Normally, cerebrospinal fluid that has been produced in the lateral and third ventricles passes through this channel to enter the fourth ventricle and so escapes through the foramina in its roof to enter the subarachnoid space. In congenital hydrocephalus, the cerebral aqueduct may be blocked or replaced by numerous small tubular passages that are insufficient for the normal flow of cerebrospinal fluid. A tumor of the midbrain (Fig. 5-33A) or pressure on the midbrain from a tumor arising outside the midbrain may compress the aqueduct and produce hydrocephalus. When the cerebral aqueduct is blocked, the accumulating cerebrospinal fluid within the third and lateral ventricles produces lesions in the midbrain. The presence of the oculomotor and trochlear nerve nuclei, together with the important descending corticospinal and corticonuclear tracts, will provide symptoms and signs that are helpful in accurately localizing a lesion in the brainstem.

Figure 5-33 Pathology of the midbrain. A: Tumor of the midbrain blocking the cerebral aqueduct. B: Weber syndrome involving the oculomotor nerve and the crus cerebri following occlusion of the blood supply to the midbrain. C: Benedikt syndrome involving the red nucleus and the medial lemniscus following occlusion of the blood supply to the midbrain.

Vascular Lesions of the Midbrain Weber Syndrome Weber syndrome (Fig. 5-33B), which is commonly produced by occlusion of a branch of the posterior cerebral artery that supplies the midbrain, results in the necrosis of brain tissue involving the oculomotor nerve and the crus cerebri. There is ipsilateral ophthalmoplegia and contralateral paralysis of the lower part of the face, the tongue, and the arm and leg. The eyeball is deviated laterally because of the paralysis of the medial rectus muscle; there is drooping (ptosis) of the upper lid, and the pupil is dilated and fixed to light and accommodation. Benedikt Syndrome Benedikt syndrome (Fig. 5-33C) is similar to Weber syndrome, but the necrosis involves the medial lemniscus and red nucleus, producing contralateral hemianesthesia and involuntary movements of the limbs of the opposite side. P.222 P.223 Clinical Problem Solving 1. While carrying out a physical examination of a patient with an intracranial tumor, the neurologist turned to a medical student and asked, “What signs or symptoms would you look for that would enable you to localize the tumor to the region of the medulla oblongata?” How would you have answered that question? View Answer1. Until involvement of one of the last four cranial nerves occurs, localization of a lesion to the medulla oblongata remains uncertain. For example, involvement of the main ascending sensory pathways or descending pathways may be caused by a lesion in the medulla, the pons, the midbrain, or the spinal cord. Involvement of the glossopharyngeal nerve can be detected by inadequacy of the gag reflex and loss of taste sensation on the posterior third of the tongue. Involvement of the vagus nerve can be assumed if the patient demonstrates some or all of the following symptoms: impairment of pharyngeal sensibility, difficulty in swallowing, nasal regurgitation of fluids with asymmetry of movement of the soft palate, and hoarseness of the voice with paralysis of the laryngeal muscles. The cranial part of the accessory nerve is distributed within the vagus nerve so that it is not possible to test for this nerve alone. The spinal part of the accessory nerve, which supplies the sternocleidomastoid and trapezius muscles, arises from the spinal cord and is therefore unaffected by tumors of the medulla. The hypoglossal nerve involvement may be tested by looking for wasting, fasciculation, and paralysis of one-half of the tongue. 2. A 6-month-old boy died with hydrocephalus and a myelocele in the lower thoracic region. At autopsy, the hindbrain was found to be deformed. The lower part of the medulla oblongata extended inferiorly through the foramen magnum into the vertebral canal as far as the third cervical vertebra. The lower four cranial nerves were longer than normal, and the upper cervical nerve roots ascended to reach their exit from the vertebral canal. The cerebellum on the left side extended inferiorly through the foramen magnum to the third cervical vertebra, where it was adherent to the spinal cord. The roof of the fourth ventricle was abnormally low. (a) What is the name of this malformation? (b) Is hydrocephalus common in this condition? (c) Is there a possible association between the thoracic myelocele and the presence of part of the hindbrain in the vertebral canal? View Answer2. (a) The malformation in which the cerebellum and the medulla oblongata are found in the cervical part of the vertebral canal is known as the Arnold-Chiari malformation. (b) Yes. Hydrocephalus is common in this condition. The hydrocephalus may be due to distortion or malformation of the openings in the roof of the fourth ventricle, which normally allow the cerebrospinal fluid to escape into the subarachnoid space. (c) Yes. A myelocele is commonly associated with this malformation. The reason for this is not exactly known, although several investigators believe that the myelocele is the primary cause and that it tethers the lower part of the spinal cord to the surrounding tissues at the time when disproportionate growth of the spinal cord and the vertebral column occurs. This would serve to pull the medulla oblongata and the cerebellum inferiorly through the foramen magnum into the vertebral canal. 3. A 68-year-old man was admitted to the hospital with the sudden onset of severe dizziness (vertigo), hiccups, and vomiting. He also complained of a hot, painful sensation in the skin of the right side of the face. On physical examination, the soft palate was drawn up to the left side when the patient was asked to say “ah,” and there was lack of mobility of the right vocal cord as seen on laryngoscopic examination. The patient also showed drooping of the right upper eyelid (ptosis), sunken right eye (enophthalmos), and a constricted right pupil (myosis). When asked to protrude his tongue straight out of his mouth, the patient tried to do so, but the tip of the tongue pointed to the right side. There was evidence of impairment of pain and temperature sensation in the trunk and extremities on the left side. Using your knowledge of anatomy, make the diagnosis. View Answer3. This patient is suffering from a thrombosis of the posterior inferior cerebellar artery or vertebral artery on the right side. The vertigo is caused by the involvement of the cerebellum or the vestibular nuclei or both. The hot, painful skin sensations are due to the involvement of the spinal tract and nucleus of the trigeminal nerve on the right side. The abnormal movement of the soft palate and the fixation of the right vocal cord are due to involvement of the nucleus of the vagus and accessory nerve on the right side. The ptosis, enophthalmos, and myosis (Horner syndrome) are due to involvement of the descending fibers of the sympathetic part of the autonomic nervous system. The pointing of the tongue to the right is caused by involvement of the right hypoglossal nucleus (the right genioglossus muscle is paralyzed). The loss of pain and temperature sensations on the opposite side of the body is due to involvement of the ascending lateral spinothalamic tracts. This characteristic clinical syndrome results from cutting off the arterial supply to a wedge-shaped area in the posterolateral part of the medulla oblongata and the inferior surface of the cerebellum. 4. A pathologist, while exploring the posterior cranial fossa during an autopsy, was endeavoring to determine where the 9th, the 10th, and the cranial part of the 11th cranial nerves emerged from the hindbrain. Describe where these nerves emerge from the hindbrain. View Answer4. The 9th, the 10th, and the cranial part of the 11th cranial nerves emerge from the medulla oblongata in a groove between the olives and the inferior cerebellar peduncles. 5. A 10-year-old girl was taken to a physician because her mother had noticed that the right half of her face was weak and did not appear to react to emotional changes. It was noted also that her mouth was pulled over slightly to the left, especially when she was tired. On questioning, the patient admitted that food tended to stick inside her right cheek and that the right side of her face “felt funny.” The mother had first noticed the facial changes 3 months previously, and the condition had progressively worsened. On examination, there was definite weakness of the facial muscles on the right side; the facial muscles on the left side were normal. Skin sensation on stimulation of the face was normal. On testing of the ocular movements, there was evidence of slight weakness of the lateral rectus muscle on the right side. Examination of the movements of the arm and leg showed slight weakness on the left side. Using your knowledge of neuroanatomy, relate these symptoms and signs to a lesion in the pons. View Answer5. This 10-year-old girl later was found to have an astrocytoma of the pons. The right unilateral facial weakness, together with weakness of the right lateral rectus muscle of the eye, was due to involvement of the right facial and abducent nuclei by the tumor. The absence of paresthesia of the face indicated that the principal sensory nucleus of the trigeminal nerve was intact on both sides. The weakness in the movements of the left arm and left leg was due to the involvement of the corticospinal fibers in the pons. (Remember that the majority of these fibers cross over to the opposite side at the decussation of the pyramids in the medulla.) 6. A 65-year-old man was admitted to the emergency department with a diagnosis of a severe pontine hemorrhage. On examination, he was found to have bilateral “pinpoint” pupils and quadriplegia. How can you explain the presence of the “pinpoint” pupils? View Answer6. “Pinpoint” pupils indicate that the constrictor pupillae muscles are strongly contracted and the dilator pupillae muscles are paralyzed. The dilator pupillae muscles are supplied by the sympathetic fibers, which descend through the pons (position not precisely known) to the lateral gray columns of the thoracic part of the spinal cord. Here, the fibers synapse, and the thoracolumbar sympathetic outflow occurs. 7. A 46-year-old man with symptoms of deafness, vertigo, and double vision (diplopia) visited his physician. On questioning, he said that he also suffered from severe headaches, which were increasing in frequency and severity. The week before, he vomited several times during one of the headache attacks. On examination, he was found to have a slight right internal strabismus, a flattening of the skin furrows on the right side of his forehead, and a slight drooping of the right corner of his mouth. There was definite evidence of impairment of hearing on the right side. On testing for sensory loss, there was definite sensory impairment on the right side of the face in the areas supplied by the maxillary and mandibular divisions of the trigeminal nerve. Using your knowledge of anatomy, explain the symptoms and signs. View Answer7. The deafness and vertigo were due to lesions in the cochlear and vestibular nuclei in the upper part of the pons. The double vision (diplopia) was produced by the involvement of the abducent nerve nucleus on the right side of the pons. The history of severe headaches and vomiting was due to a progressive rise in intracranial pressure caused by a tumor of the pons. The right unilateral facial palsy was due to the involvement of the right facial nerve nucleus. The sensory impairment of the skin of the middle and lower part of the right side of the face was due to the tumor involvement of the principal sensory nucleus of the right trigeminal nerve. 8. After a severe automobile accident that resulted in the death of the driver of one of the vehicles, an autopsy was performed, and the skull was opened. A massive subdural hematoma was found in the middle cranial fossa. The rapid accumulation of blood within the skull had exerted pressure on the brain above the tentorium cerebelli. The uncus of the temporal lobe had been forced inferiorly through the hiatus in the tentorium cerebelli. What effect do you think these intracranial changes had on the midbrain of this patient? View Answer8. The herniated uncus and the subdural hemorrhage caused pressure of the opposite crus cerebri of the midbrain against the sharp edge of the tentorium. The distortion of the midbrain caused narrowing of the cerebral aqueduct, further raising the supratentorial pressure by blocking the passage of cerebrospinal fluid from the third to the fourth ventricle. Under these circumstances, severe hemorrhage may occur within the midbrain and affect the third and fourth cranial nerve nuclei and various important descending and ascending tracts. 9. A 3-month-old girl was taken to a pediatrician because her mother was concerned about the large size of her head. The child was perfectly normal in every other respect. Examination of the child showed that the diameter of the head was larger than normal for the age; the fontanelles were larger than normal and were moderately tense. The scalp was shiny, and the scalp veins were dilated. The eyes were normal, and the mental and physical development of the child was within normal limits. CT and MRI of the head revealed gross dilation of the third and lateral ventricles of the brain. What is your diagnosis? What possible treatment should be suggested to the mother of this child? View Answer9. This child had hydrocephalus. The physical examination and the special tests showed that the third and lateral ventricles of the brain were grossly dilated owing to the accumulation of cerebrospinal fluid in these cavities. Mechanical obstruction to the flow of cerebrospinal fluid from the third into the fourth ventricle through the cerebral aqueduct was present. After the possibility of the presence of cysts or resectable tumors had been excluded, it was assumed that the cause of the obstruction was a congenital atresia or malformation of the cerebral aqueduct. If the condition were progressing—that is, the block in the aqueduct was complete and the head continued to increase in size at an abnormal rate—some form of neurosurgical procedure should have been performed whereby the cerebrospinal fluid would be shunted from the third or lateral ventricles into the subarachnoid space or into the venous system of the neck. 10. A 20-year-old man was seen by a neurologist because he had a 3-month history of double vision. On examination of the patient, both eyes at rest were turned downward and laterally. The patient was unable to move the eyes upward or medially. Both upper lids were drooping (ptosis). Examination of both pupils showed them to be dilated, and they did not constrict when a light was shone into either eye. Facial movements and sensation were normal. Movements of the upper and lower limbs were normal. There was no evidence of loss of or altered skin sensations in the upper or the lower limbs. Using your knowledge of neuroanatomy, make a diagnosis and accurately locate the site of the lesion. Is the lesion unilateral or bilateral? View Answer10. Two years later, the patient died. At autopsy, a large astrocytoma that involved the central part of the tegmentum at the level of the superior colliculi was found. The patient had exhibited all signs and symptoms associated with a raised intracranial pressure. The raised pressure was due in part to the expanding tumor, but the problem was compounded by the developing hydrocephalus resulting from blockage of the cerebral aqueduct. The symptoms and signs exhibited by the patient when he was first seen by the neurologist could be explained by the presence of the tumor in the central gray matter at the level of the superior colliculi and involving the third cranial nerve nuclei on both sides. This resulted in bilateral ptosis; bilateral ophthalmoplegia; and bilateral fixed, dilated pupils. The resting position of the eyes in a downward and lateral position was due to the action of the superior oblique muscle (trochlear nerve) and lateral rectus muscle (abducent nerve). 11. A 57-year-old man with hypertension was admitted to the hospital with a diagnosis of hemorrhage into the midbrain, possibly from a branch of the posterior cerebral artery. He was found, on physical examination, to have paralysis on the right side of the levator palpebrae superioris, the superior rectus, medial rectus, inferior rectus, and inferior oblique muscles. Furthermore, his right pupil was dilated and failed to constrict on exposure to light or on accommodation. The left eye was normal in every respect. He displayed hypersensitivity to touch on the skin of the left side of his face and had loss of skin sensation on the greater part of his left arm and left leg. The left leg also displayed some spontaneous slow writhing movements (athetosis). Using your knowledge of neuroanatomy, explain the signs and symptoms exhibited by this patient. View Answer11. The patient had a hemorrhage in the right side of the tegmentum of the midbrain that involved the right third cranial nerve. The ascending tracts of the left trigeminal nerve also were involved. After emerging from the sensory nuclei of the left trigeminal nerve, they cross the midline and ascend through the trigeminal lemniscus on the right side. The loss of sensation seen in the left upper and lower limbs was due to involvement of the right medial and spinal lemnisci. The athetoid movements of the left leg could be explained on the basis of the involvement of the right red nucleus. The absence of spasticity of the left arm and leg would indicate that the lesion did not involve the right descending tracts. For further clarification, consult the descriptions of the various tracts (see pp. 167 and 168). 12. A 41-year-old woman was diagnosed as having a lesion in the midbrain. Physical examination revealed an oculomotor nerve palsy on the left side (paralysis of the left extraocular muscles except the lateral rectus and the superior oblique muscles) and an absence of the light and accommodation reflexes on the left side. There was some weakness but no atrophy of the muscles of the lower part of the face and the tongue on the right side. There was evidence of spastic paralysis of the right arm and leg. There was no evidence of any sensory loss on either side of the head, trunk, or limbs. Using your knowledge of neuroanatomy, precisely place the lesion in the midbrain of this patient. View Answer12. Autopsy later revealed a vascular lesion involving a branch of the posterior cerebral artery. Considerable brain softening was found in the region of the substantia nigra and crus cerebri on the left side of the midbrain. The left oculomotor nerve was involved as it passed through the infarcted area. The corticonuclear fibers that pass to the facial nerve nucleus and the hypoglossal nucleus were involved as they descended through the left crus cerebri (they cross the midline at the level of the nuclei). The corticospinal fibers on the left side were also involved (they cross in the medulla oblongata), hence the spastic paralysis of the right arm and leg. The left trigeminal and left medial lemnisci were untouched, which explains the absence of sensory changes on the right side of the body. This is a good example of Weber syndrome. P.224 P.225 P.226 P.227 P.228 Review Questions Directions: Each of the numbered items in this section is followed by answers. Select the ONE lettered answer that is CORRECT. 1. The following statements concern the anterior surface of the medulla oblongata: (a) The pyramids taper inferiorly and give rise to the decussation of the pyramids. (b) On each side of the midline, there is an ovoid swelling called the olive, which contains the corticospinal fibers. (c) The hypoglossal nerve emerges between the olive and the inferior cerebellar peduncle. (d) The vagus nerve emerges between the pyramid and the olive. (e) The abducent nerve emerges between the pons and the midbrain. View Answer1. A is correct. The pyramids of the medulla oblongata taper inferiorly and give rise to the decussation of the pyramids (see Fig. 5-9). B. On each side of the midline on the anterior surface of the medulla lateral to the pyramids, there is an ovoid swelling called the olive, which contains the olivary nucleus and does not contain the corticospinal fibers (see p. 000). C. The hypoglossal nerve emerges between the pyramid and the olive. D. The vagus nerve emerges between the olive and the inferior cerebellar peduncle. E. The abducent nerve emerges between the pons and the medulla oblongata (see Fig. 5-9). 2. The following general statements concern the medulla oblongata: (a) The caudal half of the floor of the fourth ventricle is formed by the rostral half of the medulla. (b) The central canal extends throughout the length of the medulla oblongata. (c) The nucleus gracilis is situated beneath the gracile tubercle on the anterior surface of the medulla. (d) The decussation of the medial lemnisci takes place in the rostral half of the medulla. (e) The cerebellum lies anterior to the medulla. View Answer2. A is correct. The caudal half of the floor of the fourth ventricle is formed by the rostral half of the medulla oblongata (see Fig. 5-9). B. The central canal in the medulla oblongata is limited to the caudal half. C. The nucleus gracilis is situated beneath the gracile tubercle on the posterior surface of the medulla. D. The decussation of the medial lemnisci takes place in the caudal half of the medulla. E. The cerebellum lies posterior to the medulla. 3. The following statements concern the interior of the lower part of the medulla: (a) The decussation of the pyramids represents the crossing over from one side of the medulla to the other of one-quarter of the corticospinal fibers. (b) The central canal of the spinal cord is not continuous upward into the medulla. (c) The substantia gelatinosa is not continuous with the nucleus of the spinal tract of the trigeminal nerve. (d) The medial lemniscus is formed by the anterior spinothalamic tract and the spinotectal tract. (e) The internal arcuate fibers emerge from the nucleus gracilis and nucleus cuneatus. View Answer3. E is correct. The internal arcuate fibers emerge from the nucleus gracilis and nucleus cuneatus (see Fig. 4-16). A. The decussation of the pyramids represents the crossing over from one side of the medulla to the other of three-fourths of the corticospinal fibers. B. The central canal of the spinal cord is continuous upward into the medulla. C. The substantia gelatinosa becomes continuous with the nucleus of the spinal part of the trigeminal nerve. D. The medial lemniscus is formed by the axons of cells in the nucleus gracilis and the nucleus cuneatus; the axons leave the nuclei and cross the midline as the internal arcuate fibers and then ascend to the thalamus (see Fig. 4-16). 4. The following statements concern the interior of the upper part of the medulla: (a) The reticular formation consists of nerve fibers, and there are no nerve cells. (b) The nucleus ambiguus constitutes the motor nucleus of the vagus, cranial part of the accessory, and hypoglossal nerves. (c) Beneath the floor of the fourth ventricle are located the dorsal nucleus of the vagus and the vestibular nuclei. (d) The medial longitudinal fasciculus is a bundle of ascending fibers on each side of the midline. (e) The inferior cerebellar peduncle connects the pons to the cerebellum. View Answer4. C is correct. Beneath the floor of the fourth ventricle are located the dorsal nucleus of the vagus and the vestibular nuclei (see Fig. 5-14). A. The reticular formation in the upper part of the medulla oblongata consists of a mixture of nerve fibers and small nerve cells. B. The nucleus ambiguus constitutes the motor nucleus of the glossopharyngeal, vagus, and the cranial part of the accessory nerves. D. The medial longitudinal fasciculus is a bundle of ascending and descending fibers that lie posterior to the medial lemniscus on each side of the midline (see Fig. 5-14). E. The inferior cerebellar peduncle connects the medulla to the cerebellum. 5. The following statements concern the Arnold-Chiari phenomenon: (a) It is an acquired anomaly. (b) The exits in the roof of the fourth ventricle may be blocked. (c) The cerebellum never herniates through the foramen magnum. (d) It is not associated with various forms of spina bifida. (e) It is safe to perform a spinal tap in this condition. View Answer5. B is correct. In the Arnold-Chiari phenomenon, the exits in the roof of the fourth ventricle may be blocked (see p. 217). A. It is a congenital anomaly. C. The tonsil of the cerebellum may herniate through the foramen magnum (see Fig. 5-30). D. The Arnold-Chiari phenomenon is commonly associated with various forms of spina bifida. E. It is dangerous to perform a spinal tap in this condition (see p. 217). 6. The following statements concern the medial medullary syndrome: (a) The tongue is paralyzed on the contralateral side. (b) There is ipsilateral hemiplegia. (c) There are ipsilateral impaired sensations of position and movement. (d) It is commonly caused by thrombosis of a branch of the vertebral artery to the medulla oblongata. (e) There is contralateral facial paralysis. View Answer6. D is correct. The medial medullary syndrome is commonly caused by thrombosis of a branch of the vertebral artery to the medulla oblongata (see p. 218). A. The tongue is paralyzed on the ipsilateral side (see p. 218). B. There is contralateral hemiplegia. C. There are contralateral impaired sensations of position and movement. E. There is no facial paralysis. 7. The following statements concern the lateral medullary syndrome: (a) The condition may be caused by a thrombosis of the anterior inferior cerebellar artery. (b) The nucleus ambiguus of the same side may be damaged. (c) There may be analgesia and thermoanesthesia on the contralateral side of the face. (d) Contralateral trunk and extremity hypalgesia and thermoanesthesia may occur. (e) There may be evidence of seizures. View Answer7. B is correct. In the lateral medullary syndrome, the nucleus ambiguus of the same side may be damaged (see p. 217). A. The condition may be caused by thrombosis of the posterior inferior cerebellar artery. C. There may be analgesia and thermoanesthesia on the ipsilateral side of the face. D. Ipsilateral trunk and extremity hypalgesia and thermoanesthesia may occur. E. Seizures usually do not occur. Directions: Matching Questions. The following questions apply to Figure 5-34. Match the numbers listed on the left with the appropriate lettered structure listed on the right. Each lettered option may be selected once, more than once, or not at all. 8. Number 1 (a) Inferior cerebellar peduncle (b) Medial lemniscus (c) Hypoglossal nucleus (d) Reticular formation (e) None of the above View Answer8. C is correct. 9. Number 2 (a) Inferior cerebellar peduncle (b) Medial lemniscus (c) Hypoglossal nucleus (d) Reticular formation (e) None of the above View Answer9. E is correct. The structure is the medial longitudinal fasciculus. 10. Number 3 (a) Inferior cerebellar peduncle (b) Medial lemniscus (c) Hypoglossal nucleus (d) Reticular formation (e) None of the above View Answer10. B is correct. 11. Number 4 (a) Inferior cerebellar peduncle (b) Medial lemniscus (c) Hypoglossal nucleus (d) Reticular formation (e) None of the above View Answer11. E is correct. The structure is the inferior olivary nucleus. 12. Number 5 (a) Inferior cerebellar peduncle (b) Medial lemniscus (c) Hypoglossal nucleus (d) Reticular formation (e) None of the above View Answer12. D is correct. 13. Number 6 (a) Inferior cerebellar peduncle (b) Medial lemniscus (c) Hypoglossal nucleus (d) Reticular formation (e) None of the above View Answer13. A is correct. Directions: Each of the numbered items in this section is followed by answers. Select the ONE lettered answer that is CORRECT. 14. The following statements concern the pons: (a) The trigeminal nerve emerges on the lateral aspect of the pons. (b) The glossopharyngeal nerve emerges on the anterior aspect of the brainstem in the groove between the pons and the medulla oblongata. (c) The basilar artery lies in a centrally placed groove on the anterior aspect of the pons. (d) Many nerve fibers present on the posterior aspect of the pons converge laterally to form the middle cerebellar peduncle. (e) The pons forms the lower half of the floor of the fourth ventricle. View Answer14. C is correct. The basilar artery lies in a centrally placed groove on the anterior aspect of the pons (see p. 206). A. The trigeminal nerve emerges on the anterior aspect of the pons. B. The glossopharyngeal nerve emerges on the anterior aspect of the medulla oblongata in the groove between the olive and the inferior cerebellar peduncle (see Fig. 5-9). D. It is the nerve fibers on the anterior aspect of the pons that converge laterally to form the middle cerebellar peduncle. E. The pons forms the upper half of the floor of the fourth ventricle (see Fig. 5-18).

Figure 5-34 Photomicrograph of transverse section of the medulla oblongata. (Weigert stain.)

15. The following important structures are located in the brainstem at the level stated: (a) The red nucleus lies within the midbrain. (b) The facial colliculus lies in the cranial part of the pons. (c) The motor nucleus of the trigeminal nerve lies within the caudal part of the pons. (d) The abducent nucleus lies within the cranial part of the pons. (e) The trochlear nucleus lies within the midbrain at the level of the superior colliculus. View Answer15. A is correct. The red nucleus lies within the midbrain (see Fig. 5-25). B. The facial colliculus lies in the caudal part of the pons (see Fig. 5-18). C. The motor nucleus of the trigeminal nerve lies within the cranial part of the pons (see Fig. 5-20). D. The abducent nucleus lies within the caudal part of the pons (see Fig. 5-19). E. The trochlear nucleus lies within the midbrain at the level of the inferior colliculus (see Fig. 5-25). 16. The following statements concern the posterior surface of the pons: (a) Lateral to the median sulcus is an elongated swelling called the lateral eminence. (b) The facial colliculus is produced by the root of the facial nerve winding around the nucleus of the abducent nerve. (c) The floor of the inferior part of the sulcus limitans is pigmented and is called the substantia ferruginea. (d) The vestibular area lies medial to the sulcus limitans. (e) The cerebellum lies anterior to the pons. View Answer16. B is correct. On the posterior surface of the pons is the facial colliculus, which is produced by the root of the facial nerve winding around the nucleus of the abducent nerve (see Fig. 5-19). A. The medial eminence is an elongated swelling lateral to the median sulcus (see Fig. 5-26). C. The floor of the superior part of the sulcus limitans is pigmented and is called the substantia ferruginea (see Fig. 5-18). D. The vestibular area lies lateral to the sulcus limitans (see Fig. 5-18). E. The cerebellum lies posterior to the pons. 17. The following statements concern a transverse section through the caudal part of the pons: (a) The pontine nuclei lie between the transverse pontine fibers. (b) The vestibular nuclei lie medial to the abducent nucleus. (c) The trapezoid body is made up of fibers derived from the facial nerve nuclei. (d) The tegmentum is the part of the pons lying anterior to the trapezoid body. (e) The medial longitudinal fasciculus lies above the floor of the fourth ventricle on either side of the midline. View Answer17. A is correct. The pontine nuclei lie between the transverse pontine fibers (see Fig. 5-12). B. The vestibular nuclei lie lateral to the abducent nucleus (see Fig. 5-19). C. The trapezoid body is made up of fibers derived from the cochlear nuclei and the nuclei of the trapezoid body (see p. 208). D. The tegmentum is the part of the pons lying posterior to the trapezoid body. E. The medial longitudinal fasciculus lies below the floor of the fourth ventricle on either side of the midline (see Fig. 5-19). 18. The following statements concern a transverse section through the cranial part of the pons: (a) The motor nucleus of the trigeminal nerve lies lateral to the main sensory nucleus in the tegmentum. (b) The medial lemniscus has rotated so that its long axis lies vertically. (c) Bundles of corticospinal fibers lie among the transverse pontine fibers. (d) The medial longitudinal fasciculus joins the thalamus to the spinal nucleus of the trigeminal nerve. (e) The motor root of the trigeminal nerve is much larger than the sensory root. View Answer18. C is correct. In the pons, bundles of corticopontine fibers lie among the transverse pontine fibers (see Fig. 5-19). A. The motor nucleus of the trigeminal nerve lies medial to the main sensory nucleus in the tegmentum of the pons (see Fig. 5-20). B. In the cranial part of the pons, the medial lemniscus has rotated so that its long axis lies transversely (see Fig. 5-20). D. The medial longitudinal fasciculus is the main pathway that connects the vestibular and cochlear nuclei with the nuclei controlling the extraocular muscles (oculomotor, trochlear, and abducent nuclei). E. The motor root of the trigeminal nerve is much smaller than the sensory root. 19. The following statements concern the pons: (a) It is related superiorly to the dorsum sellae of the sphenoid bone. (b) It lies in the middle cranial fossa. (c) Glial tumors of the pons are rare. (d) The corticopontine fibers terminate in the pontine nuclei. (e) The pons receives its blood supply from the internal carotid artery. View Answer19. D is correct. In the pons, the corticopontine fibers terminate in the pontine nuclei (see p. 208). A. The pons is related anteriorly to the dorsum sellae of the sphenoid bone. B. The pons lies in the posterior cranial fossa. C. Astrocytoma of the pons is the most common tumor of the brainstem. E. The pons receives its blood supply from the basilar artery. Directions: Matching Questions. The following questions apply to Figure 5-35. Match the numbers listed on the left with the appropriate lettered structure listed on the right. Each lettered option may be selected once, more than once, or not at all. 20. Number 1 (a) Basilar groove (b) Medial longitudinal fasciculus (c) Superior cerebellar peduncle (d) Superior medullary velum (e) None of the above View Answer20. D is correct. 21. Number 2 (a) Basilar groove (b) Medial longitudinal fasciculus (c) Superior cerebellar peduncle (d) Superior medullary velum (e) None of the above View Answer21. C is correct. 22. Number 3 (a) Basilar groove (b) Medial longitudinal fasciculus (c) Superior cerebellar peduncle (d) Superior medullary velum (e) None of the above View Answer22. E is correct. The structure is the transverse pontine fibers. 23. Number 4 (a) Basilar groove (b) Medial longitudinal fasciculus (c) Superior cerebellar peduncle (d) Superior medullary velum (e) None of the above View Answer23. A is correct. 24. Number 5 (a) Basilar groove (b) Medial longitudinal fasciculus (c) Superior cerebellar peduncle (d) Superior medullary velum (e) None of the above View Answer24. E is correct. The structure is the medial lemniscus. 25. Number 6 (a) Basilar groove (b) Medial longitudinal fasciculus (c) Superior cerebellar peduncle (d) Superior medullary velum (e) None of the above View Answer25. B is correct. Directions: Each of the numbered items in this section is followed by answers. Select the ONE lettered answer that is CORRECT. 26. The following statements concern the midbrain: (a) It passes superiorly between the fixed and free borders of the tentorium cerebelli. (b) The oculomotor nerve emerges from the posterior surface below the inferior colliculi. (c) The superior brachium passes from the superior colliculus to the medial geniculate body. (d) The cavity of the midbrain is called the cerebral aqueduct (e) The interpeduncular fossa is bounded laterally by the cerebellar peduncles. View Answer26. D is correct. The cavity of the midbrain is called the cerebral aqueduct (see Fig. 5-28). A. The midbrain passes superiorly through the opening in the tentorium cerebelli posterior to the dorsum sellae. B. The oculomotor nerve emerges from the anterior surface of the midbrain at the level of the superior colliculi (see Fig. 5-25). C. The superior brachium passes from the superior colliculus to the lateral geniculate body and the optic tract and is associated with visual functions (see Fig. 5-23). E. The interpeduncular fossa is bounded laterally by the crus cerebri (see Fig. 5-25). 27. The following statements concern the midbrain: (a) The oculomotor nucleus is found within it at the level of the inferior colliculus. (b) The trochlear nerve emerges on the anterior surface of the midbrain and decussates completely in the superior medullary velum. (c) The trochlear nucleus is situated in the central gray matter at the level of the inferior colliculus. (d) The lemnisci are situated medial to the central gray matter. (e) The trigeminal lemniscus lies anterior to the medial lemniscus. View Answer27. C is correct. The trochlear nucleus is situated in the central gray matter of the midbrain at the level of the inferior colliculus (see Fig. 5-25). A. In the midbrain, the oculomotor nucleus is found at the level of the superior colliculus (see Fig. 5-25). B. The trochlear nerve emerges on the posterior surface of the midbrain and decussates completely in the superior medullary velum (see Fig. 5-25). D. The lemnisci are situated lateral to the central gray matter (see Fig. 5-25). E. The trigeminal lemniscus lies posterior to the medial lemniscus (see Fig. 5-25).

Figure 5-35 Photomicrograph of transverse section of the pons. (Weigert stain.)

28. The following statements concern the internal structures of the midbrain: (a) The tectum is the part situated posterior to the cerebral aqueduct. (b) The crus cerebri on each side lies posterior to the substantia nigra. (c) The tegmentum lies anterior to the substantia nigra. (d) The central gray matter encircles the red nuclei. (e) The reticular formation is limited to the lower part of the midbrain. View Answer28. A is correct. The tectum is the part of the midbrain situated posterior to the cerebral aqueduct (see Fig. 5-24). B. In the midbrain, the crus cerebri lies anterior to the substantia nigra (see Fig. 5-25). C. The tegmentum lies posterior to the substantia nigra (see Fig. 5-25). D. The central gray matter encircles the cerebral aqueduct (see Fig. 5-25). E. The reticular formation is present throughout the midbrain (see pp. 212 and 216). 29. The following statements concern the colliculi of the midbrain: (a) They are located in the tegmentum. (b) The superior colliculi are concerned with sight reflexes. (c) The inferior colliculi lie at the level of the oculomotor nerve nuclei. (d) The inferior colliculi are concerned with reflexes of smell. (e) The superior colliculi lie at the level of the trochlear nuclei. View Answer29. B is correct. The superior colliculi of the midbrain are concerned with site reflexes (see p. 212). A. The colliculi are located in the tectum (see Fig. 5-25). C. The inferior colliculi lie at the level of the trochlear nerve nuclei (see Fig. 5-25). D. The inferior colliculi are concerned with auditory reflexes. E. The superior colliculi lie at the level of the red nuclei (see Fig. 5-25). 30. The following statements concern the third cranial nerve nuclei: (a) The oculomotor nucleus is situated lateral to the central gray matter. (b) The sympathetic part of the oculomotor nucleus is called the Edinger-Westphal nucleus. (c) The oculomotor nucleus lies posterior to the cerebral aqueduct. (d) The nerve fibers from the oculomotor nucleus pass through the red nucleus. (e) The oculomotor nucleus lies close to the lateral longitudinal fasciculus. View Answer30. D is correct. The nerve fibers from the oculomotor nucleus pass through the red nucleus (see Fig. 5-25). A. The oculomotor nucleus is situated in the central gray matter (see Fig. 5-25). B. The parasympathetic part of the oculomotor nucleus is called the Edinger-Westphal nucleus. C. The oculomotor nucleus lies anterior to the cerebral aqueduct (see Fig. 5-25). E. The oculomotor nucleus lies close to the medial longitudinal fasciculus (see Fig. 5-25). Directions: Matching Questions. The following questions apply to Figure 5-36. Match the numbers listed on the left with the appropriate lettered structure listed on the right. Each lettered option may be selected once, more than once, or not at all. 31. Number 1 (a) Medial longitudinal fasciculus (b) Inferior colliculus (c) Medial lemniscus (d) Trochlear nucleus (e) None of the above View Answer31. E is correct. The structure is the superior colliculus. 32. Number 2 (a) Medial longitudinal fasciculus (b) Inferior colliculus (c) Medial lemniscus (d) Trochlear nucleus (e) None of the above View Answer32. E is correct. The structure is the oculomotor nucleus. 33. Number 3 (a) Medial longitudinal fasciculus (b) Inferior colliculus (c) Medial lemniscus (d) Trochlear nucleus (e) None of the above View Answer33. E is correct. The structure is decussation of the rubrospinal tract. 34. Number 4 (a) Medial longitudinal fasciculus (b) Inferior colliculus (c) Medial lemniscus (d) Trochlear nucleus (e) None of the above View Answer34. E is correct. The structures are corticospinal and corticonuclear fibers. 35. Number 5 (a) Medial longitudinal fasciculus (b) Inferior colliculus (c) Medial lemniscus (d) Trochlear nucleus (e) None of the above View Answer35. C is correct. 36. Number 6 (a) Medial longitudinal fasciculus (b) Inferior colliculus (c) Medial lemniscus (d) Trochlear nucleus (e) None of the above View Answer36. E is correct. The structure is the mesencephalic nucleus of V.

Figure 5-36 Transverse section of the midbrain.

Directions: Each case history is followed by questions. Read the case history, then select the ONE BEST lettered answer. A 63-year-old man complaining of difficulty in swallowing, some hoarseness of his voice, and giddiness was seen by a neurologist. All these symptoms started suddenly 4 days previously. On physical examination, he was found to have a loss of the pharyngeal gagging reflex on the left side, left-sided facial analgesia, and left-sided paralysis of the vocal cord. 37. Based on the clinical history and the results of the physical examination, select the most likely diagnosis. (a) A meningeal tumor in the posterior cranial fossa on the right side (b) Lateral medullary syndrome on the left side (c) Medial medullary syndrome on the left side (d) Lateral medullary syndrome on the right side (e) Medial medullary syndrome on the right side View Answer37. B is correct (see p. 217). A 7-year-old girl was seen by a neurologist because she complained to her mother that she was seeing double. Careful physical examination revealed that the double vision became worse when she looked toward the left. The patient also had evidence of a mild motor paralysis of her right lower limb without spasticity. There was also a slight facial paralysis involving the whole left side of the face. 38. Based on the clinical history and the clinical examination, the following neurologic deficits could have been present except: (a) The double vision caused by weakness of the left lateral rectus muscle. (b) The complete left-sided facial paralysis caused by involvement of the left seventh cranial nerve nucleus or its nerve. (c) The mild right hemiparesis produced by damage to the corticospinal tract on the right side. (d) An MRI revealed the presence of a tumor of the lower part of the pons on the left side. (e) There was damage to the left sixth cranial nerve nucleus. View Answer38. C is correct. The right-sided hemiparesis was caused by damage to the corticospinal tract on the left side of the pons. The corticospinal tract descends through the medulla and crosses to the right side of the midline at the decussation of the pyramids. The patient was later discovered to have a glioma involving the left side of the lower pons. A 42-year-old woman complaining of a severe, persistent headache visited her physician. At first, the headache was not continuous and tended to occur during the night. Now, the headache was present all the time and was felt over the whole head. Recently, she has begun to feel nauseous, and this has resulted in several episodes of vomiting. Last week, on looking in the mirror, she noted that her right pupil looked much larger than the left. Her right upper lid appeared to droop. 39. The physical examination revealed the following most likely findings except: (a) There was weakness in raising the right eyelid upward. (b) There was severe ptosis of the right eye. (c) There was obvious dilatation of the right pupil. (d) Ophthalmoscopic examination revealed bilateral papilledema. (e) There was no evidence of paralysis of either superior oblique muscle. (f) Examination of the lower limbs revealed a mild spasticity of the left lower limb muscles. (g) Ataxia of the right upper limb was also present. (h) There was a loss of taste sensation on the posterior one-third of the tongue on the left side. View Answer39. H is correct. The sensation of taste on the posterior one-third of the tongue is supplied by the glossopharyngeal nerve, which originates in the medulla oblongata. 40. The combination of the clinical history and the findings in the physical examination enabled the physician to make the following most likely diagnosis. (a) A tumor involving the left cerebral hemisphere (b) A tumor involving the right side of the midbrain at the level of the superior colliculi (c) Severe migraine (d) A cerebral hemorrhage involving the left cerebral hemisphere (e) A tumor of the left side of the midbrain View Answer40. B is correct. The combination of raised intracranial pressure (headache, vomiting, and bilateral papilledema), the involvement of the right third cranial nerve (right-sided ptosis, right pupillary dilatation, and right-sided weakness of ocular deviation upward), spasticity of the left leg (right-sided corticospinal tracts), and ataxia of the right upper limb (cerebellar connections on the right side) led the physician to make a tentative diagnosis of an intracranial tumor in the right side of the midbrain at the level of the superior colliculi. An MRI confirmed the diagnosis. P.229 Additional Reading Brazis, P. W., Masdeu, J. C., and Biller, J. Localization in Clinical Neurology (2nd ed.). Boston: Little, Brown, 1990. Brodal, A. Neurological Anatomy in Relation to Clinical Medicine (3rd ed.). New York: Oxford University Press, 1981. Crosby, E. C., Humphrey, T., and Lauer, E. W. Correlative Anatomy of the Nervous System. New York: Macmillan, 1962. Goetz, C. G. Textbook of Clinical Neurology. Philadelphia: Saunders, 2003. Martin, J. H. Neuroanatomy: Text and Atlas (2nd ed.). Stamford, CT: Appleton & Lange, 1996. Patten, J. P. Neurological Differential Diagnosis. London: Harold Stark, 1980. Paxinos, G. The Human Nervous System. San Diego: Academic Press, 1990. Rowland, L. P. (ed.). Merritt’s Textbook of Neurology (10th ed.). Philadelphia: Lippincott Williams & Wilkins, 2000. Snell, R. S. Clinical Anatomy by Regions (8th ed.). Philadelphia: Lippincott Williams & Wilkins, 2008. Snell, R. S. Clinical Anatomy by Systems. Philadelphia: Lippincott Williams & Wilkins, 2007. Standring, S. (ed.). Gray’s Anatomy (39th Br. ed.). London: Elsevier Churchill Livingstone, 2005. Walton, J. N. Brain’s Diseases of the Nervous System (9th ed.). New York: Oxford University Press, 1984.

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Gamal Hassan 28-12-2015, 07:00

A very good and valuable text

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