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RECTUM

The rectum is continuous with the sigmoid colon at the level of the third sacral vertebra and terminates at the upper end of the anal canal. It descends along the sacrococcygeal concavity as the sacral flexure of the rectum, initially inferoposteriorly and then inferoanteriorly, to join the anal canal by passing through the pelvic diaphragm. The anorectal junction is 2–3 cm in front of and slightly below the tip of the coccyx, which is opposite the apex of the prostate in males. From this level the anal canal passes inferiorly and posteriorly from the lower end of the rectum. The posterior bend is termed the perineal flexure of the rectum and the angle it forms with the upper anal canal is the anorectal angle. The rectum also deviates in three lateral curves: upper, convex to the right; middle (the most prominent), convex to the left; lower, convex to the right. Both ends of the rectum are in the median plane (Fig. 67.34).

  

Fig. 67.34  A, Coronal T2-weighted MRI of the rectum. B, Line diagram illustrating main features in A.

Although variable in absolute length, a common landmark used in clinical practice to define the rectum is a length of 15 cm above the external anal margin. The initial diameter is similar to that of the sigmoid colon, but more inferiorly it becomes dilated as the rectal ampulla. The rectum differs from the sigmoid colon in having no sacculations or appendices epiploicae. The taeniae blend approximately 5 cm above the rectosigmoid junction, forming two wide muscular bands which descend anteriorly and posteriorly in the rectal wall. These then fuse to form an encircling layer of longitudinal muscle, which invests the entire length of the rectum. At the rectal ampulla a few strands of the anterior longitudinal fibres pass forwards to the perineal body as the musculus recto-urethralis. In addition, two fasciculi of smooth muscle may pass anteroinferiorly from the anterior surfaces of the bodies of the second and third coccygeal vertebrae to blend with the longitudinal muscle fibres on the posterior wall of the anal canal, forming the rectococcygeal muscles.

The upper third of the rectum is covered by peritoneum on its anterior and lateral aspects. It is related anteriorly to the sigmoid colon or loops of ileum if these lie in the pelvis, otherwise it is related to the urinary bladder in males, or the cervix and body of the uterus in females. The middle third of the rectum is covered by peritoneum only on its anterior aspect. The peritoneum is reflected superiorly onto the urinary bladder in males to form the rectovesical pouch, or onto the posterior vaginal wall in females to form the recto-uterine pouch (pouch of Douglas). The level of this reflection is higher in males; the rectovesical pouch is approximately 7.5 cm (about the length of the index finger) from the anorectal junction. In females the recto-uterine pouch is approximately 5.5 cm from the anorectal junction. In the male neonate, peritoneum extends on to the front of the rectum as far as the lower limit of the prostate. Superiorly the peritoneum is firmly attached to the muscle layer of the sigmoid colon by fibrous connective tissue, but as it descends onto the rectum it is more loosely attached by fatty connective tissue, which allows for considerable expansion of the upper half of the rectum (Figs 67.35, 67.36).

  

Fig. 67.35  A, Sagittal T2-weighted MRI of the rectum in a male. B, Line diagram illustrating main features in A.

  

Fig. 67.36  A, Sagittal T2-weighted MRI of the rectum in a female. B, Line diagram illustrating main features in A.

There are no haustra in the rectum. When empty the mucosa forms a number of longitudinal folds in its lower part which become effaced during distension. The rectum commonly has three permanent semi-lunar transverse or horizontal folds (although the number can vary), which are most marked in rectal distension (Fig. 67.9). Two forms of horizontal fold have been recognized. One consists of the mucosa, a circular muscle layer and part of the longitudinal muscle, and is marked externally by an indentation. The other is devoid of longitudinal muscle and has no external marking. The most superior fold at the beginning of the rectum may be either on the left or right and occasionally encircles the rectal lumen. The middle fold is largest and most constant. It lies immediately above the rectal ampulla, projecting from the anterior and right wall just below the level of the anterior peritoneal reflection; the circular muscle is more marked in this fold than in the others. The most inferior and variable fold is found on the left below the middle fold. Sometimes a fourth fold is found on the left a little above the middle fold.

MESORECTUM, RECTAL FASCIAE AND ‘SPACES’

The mesorectum (mesentery of the rectum) and its contents are intimately related to the rectum down to the level of levator ani (Figs 67.37–67.39). It is a distinct compartment derived from the embryological hindgut. It contains the superior rectal artery and its branches, the superior rectal vein and its tributaries, the lymphatic vessels and nodes that lie along the superior rectal artery, branches from the inferior mesenteric plexus which descend to innervate the rectum, and loose adipose connective tissue.

  

Fig. 67.37  A, Axial T2-weighted MRI of the upper rectum in a female. B, Line diagram illustrating main features in A.

  

Fig. 67.38  A, Axial T2-weighted MRI of the mid rectum below the peritoneal reflection in a male. B, Line diagram illustrating main features in A.

  

Fig. 67.39  A, Axial T2-weighted MRI of the low rectum below the peritoneal reflection in a female. B, Line diagram illustrating main features in A.

The mesorectum is enclosed by mesorectal fascia, a distinct covering derived from the visceral peritoneum that is also called the visceral fascia of the mesorectum, fascia propria of the rectum or the presacral wing of the hypogastric sheath. The fascia bounds the mesorectum posteriorly and thus lies anterior to the retrorectal space and the pre-sacral fascia. The mesorectal fascia is surrounded by a very thin layer of loose areolar tissue which separates it from the posterior and lateral walls of the true pelvis. Superiorly, the mesorectal fascia blends with the connective tissue bounding the sigmoid mesentery. Laterally, it extends around the rectum and mesorectum and becomes continuous with a denser condensation of fascia anteriorly. In males this anterior fascia is known as the rectovesical fascia of Denonvillier, and in females it forms the fascia of the rectovaginal septum.

On MRI scanning, the mesorectum appears as a fat-containing envelope in which vessels are depicted as low signal due to signal void produced by blood flow. Lymph nodes appear as high signal ovoid structures. Small nerves within the mesorectum are not visualized, but interlacing connective tissue within the mesorectum can be seen as low signal intensity strands. The mesorectal fascia is demonstrated on axial views as a low signal layer that surrounds the mesorectum, and which corresponds to the distinct condensation of fascia seen on histological sections containing the mesorectum (Brown et al 1999). Identification of the involvement of this layer by malignant tumours of the rectum on MRI scanning may help plan preoperative radiotherapy, and predict the chance of successful surgical resection.

Anterolaterally, branches of the inferior hypogastric plexus and branches of the middle rectal artery and veins run into the mesorectum. They are ensheathed by fascia and are sometimes jointly referred to as the ‘lateral rectal ligaments’. The number and calibre of the middle rectal vessels are highly variable; they may be very small or even absent (Sato & Sato 1991). The ‘lateral ligaments’ are not seen on MRI or CT scanning, and only appear as an identifiable structure with surgical traction on the rectum. The fascia of the ‘ligaments’ is flimsy and they probably play very little role in support of the rectum.

The parietal fascia that covers levator ani and the muscles of the side-wall of the pelvis forms a denser condensation of fascial tissue overlying the sacrum (Fig. 67.40).

  

Fig. 67.40  A, Light microscope transverse section of the mid rectum (male, cadaveric specimen). B, Line diagram illustrating main features in A.

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