STOMACH AND INTESTINE. 



369 



it is related to the ureter, and to the branches 

 of the internal iliac vessels. 



The middle or arcuate segment (r 2, Jig. 

 276.) scarcely exceeds 2f 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 becomes 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, which is called the recto- 

 vesicul pouch (r v, Jig. 276.). The apex of 

 this, below, corresponds to the point where the 

 membrane is reflected from 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 with 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- 

 nalis (i, fig. 276.), and behind by the pouch 

 aforesaid : a space, which therefore corre- 

 sponds to the "trigone vesicate" 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 rectum, 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 coccyx. 



The third or terminal portion of the bowel 

 0' 3 '/g- 276.) is about 1| 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,fg. 276.) 

 and the membranous 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 structure 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 wholeof 

 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 

 Supp. 



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 oesophagus 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 internu-Sy which is situated within the 

 bowel, is little more than a thickening of 

 the proper transverse or circular layer 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 muscular fibre, 

 which surrounds the lower extremity of the 

 rectum (at w, Jig. 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. Hence 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 perinaeum. 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 vaginae, by what is often a distinct 

 decussation. 



It is chiefly through the intervention of this 

 sphincter that the levator 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 number 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 more posterior fibres 



* For a fuller description of the origin and rela- 

 tions of the above muscle, the reader is referred to 

 the Article PELVIS. 



B B 



