The middle or arcuate segment (r 2, fig. 276.) scarcely exceeds 2/ inches in length. It lies on the sacrum and coccyx ; to which it is attached by a loose areolar tissue, that usually encloses much fat. The peritoneum, which invests a small part of its lateral sur faces above, soon becornes limited to its anterior surface as it descends ; and ceases altogether at a distance of about an inch from the termination of this part of the bowel. This incomplete serous covering is, in fact, the lower extremity of a pro cess of peritoneum, viltich is called the recto vesical pouch (r r, fig. 276.). The apex of this, below, corresponds to the point where the rnembrane is reflected frona the anterior sur face of the rectum, to the neighbouring, pos terior surface of the bladder. Hence the front of the bowel, which, above, comes into contact with the distended bladder, or vvith any con volution of small intestine that may chance to occupy the recto-vesical pouch, touches, be low, the anterior part of the base of the blad der. This it does in a triangular space, that is bounded on each side by- the vesicula semi nails (i, fig. 276.), and behind by the pouch aforesaid :— a space which therefore corre sponds to the "trigone vesicale" on the inner surface of the bladder ; and marks a region, where this viscus may be punctured through the rectum, without any injury to the serous membrane. On each side of this part of the rectutn, is a lamina of pelvic fascia which supports it; and externally to this is the coccygeus muscle, which descends from the spine of the ischium (h, fig. 276.) to the bor der of the coccy x.
The third or terminal portion of the bowel (r 3, flg. 276.) is about inches long. It is surrounded, above, by areolar tissue loaded with fat: below, by the sphincter ani externus (at n); and by the levator ani (o and q) which descends from the pelvis to mingle its fibres of insertion with those of the former muscle. In front of it are the prostate (k, fig. 276.) and the membranou.s portion of the urethra in the male, the vagina in the female: behind it, the tip of the coccyx in both sexes. The degree in which the prostate projects into the rectum will of course vary-, both with the size of this gland, and with the amount of dis tention to which the tube itself is habitually subjected. Where its distention is very great, the flaccid intestine sometimes appears almost folded around the prostate gland, so as to offer a kind of pouch on either side of it.
As regards the strurture of the rectum, its peritoneal coat has already received a sufficient description. Its muscular tunic differs remark ably from that of the colon. In place of being divided into three bands,the fibres of the longi tudinal coat are again collected into a single uniform expanse, that surrounds the whole of the tube. And both it and the transverse layer are very much increased in thickness. Thus even in the upper part of the rectum, the muscular coat offers a total thickness which at least doubles that of the colon; while near its termination, it is scarcely less than four times as thick. The fibres themselves are of a red der colour, especially the transverse; and near the anus, they are mingled with the striated elements of the adjoining voluntary muscles.
And that distant analogy which the rectum seems to offer the cesophagus in all these respects, is rendered somewhat closer by the presence of numerous folds of mucous membrane, which the mere passive contrac tion of its muscular tunic appears generally to produce. In short, it would hardly be an exaggeration to say that this, the last segment of the digestive canal, resumes not only the median position, but even the structure and action, of the first.
Muscles of the anus.— The muscular appa ratus that guards the lower outlet of the rec tum, consists chiefly of two sphincter muscles, which close the orifice of the anus by their contraction. Of these, one, the sphincter ani interims, which is situated within the bowel, is little more than a thickening of the proper transverse or circular lay er of its muscular coat. It lies immediately beneath the mucous membrane, and consists of the ordinary unstriped fibres, which retain their usual pale colour. The other, the proper sphincter, or the sphincter ani externus, is a deep-red mass of striated nauscular fibre, which surrounds the lower extremity of the rectum (at n, A. 276.), including the pre ceding structure. Its form is elliptical and con cave; so that its outer border occupies a higher level than its inner one. Ilence it receives the end of the bowel as it were into a shallow funnel; and, on contracting, occludes its cavity by. the pressure of what is a surface, and not merely an edge, of its plane of fibres. Its outermost bundles arise from a tendinous structure which is prolonged from the tip of the coccyx; and they pass forwards, on both sides of the bowel, to reunite in the central tendinous point of the perinurn. Its inner most or lowest fibres are attached to a dense subcutaneous areolar tissue that lies before and behind the anus. In the female, they interlace anteriorly with those of the con strictor vagina., by what is often a distinct decussation.
It is chiefly through the intervention of this sphincter that the lerator ani muscle is con nected with the lower extremity of the rec tum. The fibres of this large expanse of striped muscular substance descend from their pelvic origin*, with an inclination backwards, to reach the lower orifice of the cavity of the pelvis, which they assist to close. In front of the rectum, a certain nunaber of them are inserted into the tendinous and areolar tissues which here occupy the middle line of the peri neum ; and these join more or less directly with the attachment of the external sphincter in the same situation. Behind the rectum, a considerable plane of the rnore posterior fibres of the levator passes around the tube, return ing upon itself in the similar portion from the opposite side, so as to sling and sustain the canal. Between these anterior and posterior slips, is a median portion ; which descends to be inseparably united with the upper and outer rings of the external sphincter itself. And besides interlacing with the striped fibres of this muscle, it mav generally be traced sending off a certain number of its scattered bundles, to decussate and disappear amongst the un striped fibres of the longitudinal and transverse layers of the rectum just at their termination.