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PENIS

The penis consists of an attached root (radix) in the perineum and a free, pendulous, body (corpus), which is completely enveloped in skin.

Skin

The penile skin is thin and loosely connected to the tunica albuginea. At the corona of the penis it is folded to form the prepuce or foreskin, which invariably overlaps the glans. The internal preputial layer is confluent at the neck with the thin skin covering and adhering firmly to the glans, and by this with the urethral mucosa at the external urethral orifice. On the urethral aspect of the glans a median fold, the frenulum, passes from the deep surface of the prepuce to the glans immediately proximal to the orifice. Cutaneous sensitivity is greatest over the glans penis. The prepuce and glans penis enclose a potential cleft, the preputial sac, and two shallow fossae flank the frenulum (Fig. 76.16).

  

Fig. 76.16  Structures in the male urogenital triangle. A, Inferior view. B, Ventral surface of the body of the penis. C, Lateral view of the body of the penis and glans. D, Inferior view of the urogenital triangle of a man with the erectile tissues of the penis indicated with overlays.
(From Drake, Vogl and Mitchell 2005.)

Root

The root of the penis consists of three masses of erectile tissue in the urogenital triangle, namely the two crura and the bulb, which are firmly attached to the pubic arch and perineal membrane respectively. The crura are the posterior extremities of the corpora cavernosa, and the bulb is the dilated posterior end of the corpus spongiosum.

Each penile crus starts behind as a blunt, elongated but rounded structure, attached firmly to the everted edge of the ischiopubic ramus and covered by ischiocavernosus (Fig. 76.17). Anteriorly it converges towards the contra-lateral crus and is slightly enlarged posterior to this. Near the inferior symphysial border the two crura come together and continue as the corpora cavernosa of the body of the penis.

  

Fig. 76.17  A, Erectile tissues of the penis. B, Muscles in the superficial perineal pouch. C, Muscles and erectile tissues of the penis in section.
(A and B from Drake, Vogl and Mitchell 2005; C adapted from Drake, Vogl, Mitchell, Tibbitts and Richardson 2008.)

The bulb of the penis lies between the crura and is firmly connected to the inferior aspect of the perineal membrane, from which it receives a fibrous covering (Figs 76.16D, 76.17). Oval in section, the bulb narrows anteriorly into the corpus spongiosum, down and forwards at this point. Its convex superficial surface is covered by bulbospongiosus and its flattened deep surface is pierced above its centre by the urethra, which traverses it to reach the corpus spongiosum.

Body

The main body of the penis consists of three masses of erectile tissue, the right and left corpora cavernosa, and the median corpus spongiosum, which are continuations of the crura and bulb of the penis respectively (Figs 76.17A, 76.18A,B). They become engorged with blood during penile erection (Fig. 76.19A). The penis is cylindrical when flaccid, but becomes triangular when erect. The surface which is posterosuperior during erection is termed the dorsum of the penis and the opposite aspect is the ventral surface.

  

Fig. 76.18  A, Layers of the penis and main structures. B, Cross section of the body of the penis.

  

Fig. 76.19  A,B, MRI scan of the penis showing the corpora cavernosum and spongiosum. Note the corpus spongiosum flaring posteriorly into the bulbus spongiosum. C, MRI scan showing the bulb of the penis and the attachment of the posterior portion of the corpora cavernosa, the crura. D,E, MRI scan of the penis showing the tunica albuginea.

Corpora cavernosa

The corpora cavernosa form most of the body of the penis. They share a common fibrous envelope, the tunica albuginea, and are in close apposition throughout their length, although separated by a median fibrous septum. On the urethral surface their combined mass has a wide median groove that adjoins the corpus spongiosum (Figs 76.17–76.19); dorsally a similar but narrower groove contains the dorsal neurovascular bundle. The corpora end distally within the proximal aspect of the glans penis in a rounded cone, in which each has a small terminal projection (Fig. 76.16D).

The strong fibrous tunica albuginea consists of superficial and deep strata. The superficial fibres run longitudinally, forming a single tube that surrounds both corpora, whereas the deep fibres are circularly arranged and surround each corpus separately. The deep fibres join to form the median septum of the penis. This is thick and complete proximally, so that the corporal bodies can be separated proximally for 5–7 cm. Distally the median septum consists of a pectiniform (comb-like) series of bands, the pectiniform septum, which is incomplete and allows cross-circulation of blood between the two corpora.

Corpus spongiosum

The corpus spongiosum of the penis is traversed by the urethra (Figs 76.17A, 76.18B, 76.19). It adjoins the median groove on the urethral surface of the conjoined corpora cavernosa. It is cylindrical, tapering slightly distally, and surrounded by a tunica albuginea. Near the end of the penis it expands into the glans penis which projects dorsally over the end of the corpora cavernosa and has a shallow concave surface to which they are attached. The corona glandis projects from its base, overhanging an obliquely grooved neck of the penis. Numerous small preputial glands on the corona glandis and penile neck secrete sebaceous smegma. The navicular fossa of the urethra is in the glans and opens by a sagittal slit on or near its apex.

Superficial penile fascia

The superficial penile fascia is devoid of fat, and consists of loose connective tissue invaded by fibres of the dartos muscle from the scrotum. Clinically, it is commonly referred to as the dartos layer. As in the suprapubic abdominal wall, the deepest layer is condensed to form a distinct tough fascial sheath, Buck’s fascia (Fig. 76.18B). It surrounds both corpora cavernosa and splits to enclose the corpus spongiosum, separating the superficial and deep dorsal veins. At the penile neck it blends with the fibrous covering of all three corpora. Proximally, it is continuous with the dartos muscle and with the fascia covering the urogenital region of the perineum (see p. 1094).

Suspensory ligaments of penis

The body of the penis is supported by two ligaments, the fundiform and triangular ligaments, which are continuous with its fascia and consist largely of elastin fibres (Fig. 76.17B). The fundiform ligament stems from the lowest part of the linea alba and splits into two lamellae which skirt the penis and unite below with the scrotal septum. The triangular suspensory ligament, deep to the fundiform ligament, is attached above to the front of the pubic symphysis and blends below with the fascia penis on each side. Damage to the suspensory ligament may occur after trauma and results in ventral angulation of the body of the penis.

Vascular supply and lymphatic drainage

Perineal artery

The perineal artery (Fig. 76.20) leaves the internal pudendal artery near the anterior end of its canal and approaches the scrotum in the superficial perineal region, between bulbospongiosus and ischiocavernosus. Beyond the perineal membrane, and near its base, a small transverse branch passes medially, inferior to the superficial transverse perineal muscle, to anastomose with its contralateral fellow and with the posterior scrotal and inferior rectal arteries; collectively these vessels supply tissues between the anus and the penile bulb. The posterior scrotal arteries, distributed to the scrotal skin, dartos and perineal muscles, are usually terminal branches of the perineal artery but may also arise from its transverse branch.

  

Fig. 76.20  Blood vessels and nerves of the perineal region and external genitalia in the adult male. The fat body of the ischio-anal fossa has been removed and gluteus maximus has been incised in order to expose the course of the pudendal nerve and internal pudendal artery.
(From Sobotta 2006.)

Artery of the bulb of the penis

The artery of the bulb of the penis is short but wide, and runs medially through the deep transverse perineal muscle to the penile bulb, which it penetrates (Fig. 76.20). It supplies the corpus spongiosum and the bulbourethral gland.

Cavernosal artery (deep artery of the penis)

The cavernosal artery is a terminal branch of the internal pudendal artery (Fig. 76.20). It passes through the perineal membrane, enters the crus penis on each side and runs the length of the corpus cavernosum, supplying the erectile tissue. Within the corpus, the cavernosal arteries divide into branches that either end directly in capillary networks which open into the cavernous spaces, or become convoluted and somewhat dilated helicine arteries, which also open into the cavernous spaces. Helicine arteries are most abundant in the posterior regions of the corpora cavernosa.

Dorsal artery of the penis

The dorsal artery of the penis is the other terminal branch of the internal pudendal artery (Figs 76.20, 76.21). It runs between the crus penis and pubic symphysis, and then pierces the suspensory ligament of the penis to run along its dorsum to the glans, where it forks into branches to the glans and prepuce. In the penis it lies deep to Buck’s fascia between the dorsal nerves and the deep dorsal vein, the latter being most medial. It supplies penile skin by branches that run through the dartos layer. It gives off circumflex branches that run laterally across the shaft of the penis, first deep to and then within Buck’s fascia, to supply the tunica albuginea of the corpus cavernosum, anastomosing through the tunica with the cavernosal system; these branches also supply the corpus spongiosum.

  

Fig. 76.21  Arterial blood supply to the body of the penis.

Dorsal veins of the penis

The veins that drain the corpora cavernosa leave the corpora by passing obliquely through the tunica albuginea via a series of small vessels called sub-tunical veins. These drain into the circumflex veins (Fig. 76.22) which run circumferentially around the shaft of the penis from its ventral aspect, where they receive tributaries from the corpus spongiosum, to its dorsal aspect, where they drain into the deep dorsal vein. The dorsal veins, superficial and deep, are unpaired. The superficial dorsal vein drains the prepuce and penile skin: it runs back in subcutaneous tissue and inclines either right or left before it opens into one of the external pudendal veins. The deep dorsal vein lies deep to Buck’s fascia. It receives blood from the glans penis and corpora cavernosa penis, and courses back in the midline between the paired dorsal arteries. Near the root of the penis it passes deep to the suspensory ligament and through a gap between the arcuate pubic ligament and anterior margin of the perineal membrane. It divides into right and left branches that connect below the symphysis pubis with the internal pudendal veins and ultimately enter the prostatic plexus.

  

Fig. 76.22  Perineal vessels.
(Adapted from Drake, Vogl and Mitchell 2005.)

Lymphatic drainage

The penile and perineal skin is drained by lymph vessels that accompany the external pudendal blood vessels to the superficial inguinal nodes. Lymph vessels from the glans pass to the deep inguinal and external iliac nodes. Lymph vessels from the erectile tissue and penile urethra pass to the internal iliac lymph nodes.

Innervation

The nerves to the corpora cavernosa mainly consist of the cavernous nerves, which arise from the pelvic plexus and contain both sympathetic and parasympathetic components (Fig. 76.23). The cavernous nerves pierce the fibrous penile sheath proximally to supply the erectile tissue of the corpus spongiosum and penile urethra. During their course they run postero-laterally to the apex of the prostate. The sympathetic supply to the male genital organs is derived from spinal cord segments T11–L2: stimulation produces vasoconstriction, contraction of the seminal vesicles and prostate, and seminal emission. Parasympathetic fibres are derived from spinal cord segments S2–4 via the pelvic plexuses: stimulation produces vasodilatation.

  

Fig. 76.23  The nerve supply to the penis. The corpus cavernosum of penis receives both a parasympathetic and sympathetic innervation from the cavernous nerves. The afferent fibres from the glans pass via the dorsal nerves of the penis and via the pudendal nerve.
(Adapted from Drake, Vogl and Mitchell 2005.)

The main sensory nerve of the penis is the dorsal nerve of the penis. On the glans and bulb of the penis some cutaneous filaments innervate lamellated corpuscles. Afferent fibres from the glans penis and perigenital skin pass via the pudendal nerve to the spinal cord.

Microstructure

The internal surfaces of the fibrous sheaths of the corpora cavernosa and their dividing septum give rise to numerous trabeculae. These cross the corpora cavernosa in all directions and divide them into a series of cavernous spaces, giving them a spongy appearance. The trabeculae are composed of collagen and elastin fibres and smooth muscle and contain numerous vessels and nerves. The cavernous spaces are lined by an endothelium similar to that in other blood vessels. The spaces are empty in the flaccid state but become filled with blood during erection. The fibrous tunica albuginea of the corpus spongiosum is thinner, whiter and more elastic than that of the corpora cavernosa. It is formed partly of smooth muscle cells: a layer of the same tissue surrounds the urethral epithelium and the para-urethral glands.

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