THE RECTUM 1177 



The peritoneum is reflected anteriorly from the rectum to the bladder in the 

 male (recto-vesical pouch) and to fornix of the vagina in the female (recto-vaginal 

 pouch). In the newborn, the peritoneum reaches to the base of the prostate 

 (Symington). On the posterior surface of the gut, there is no peritoneum below 

 a point about 12.5 cm. from the anus. Thus the peritoneum at the upper end of 

 the rectum entirely surrounds the gut. Lower down it covers only the sides and 

 anterior wall, and lower still the anterior wall only, where it is reflected upon the 

 bladder or vagina. 



The second portion of the rectum, or anal canal [pars analis recti] is from 2.5 

 cm. to 3.5 cm. in length. From the lower end of the first portion, it turns at right 

 angles downward and backward, passing through the pelvic floor, and ending at 

 the anus. It is entirely below the peritoneum, and is surrounded by the two 

 sphincter muscles (figs. 936, 937). 



Anteriorly is the bulb of the urethra and the posterior margin of the urogenital trigone in 

 the male (fig. 937), while in the female it is separated from the vestibule and the lower part of 

 the vagina by the 'perineal body' (fig. 938). Posteriorly it is connected with the tip of the 

 coccyx by the ano-coccygeal body. Laterally it is in contact with the margins of the levatores 

 ani, which act as an accessory sphincter, and help to support the ampulla recti. 



The anus. — The anus is the aperture by which the intestine opens externally . 

 During life it is contracted by the sphincters, so as to give the surrounding skin 

 a wrinkled appearance. Around the lower part of the rectum and anus certain 

 muscles that are connected with its proper function are situated. They are the 

 internal sphincter, the levator ani, and the external sphincter. The levator ani 

 and external sphincter will be found described in the section on Musculature. 

 The internal sphincter is a thickening of the circular fibres of the intestine, situated 

 around the second portion or anal canal. It forms a complete muscular ring, 2 to 

 3 mm. thick, and is composed of non-striated muscle. 



The rectum differs from the rest of the colon in having smoother walls and no 

 appendices epiploicse. At the upper end of the rectum, the taenia libera and taenia 

 omentalis join to form a broad band which spreads out, covering the entire anter- 

 ior aspect of the rectum. Similarly the taenia mesocolica spreads out upon the 

 posterior aspect. Thus the rectum has a complete longitudinal muscle layer, 

 which, however, is thicker anteriorly and posteriorly than laterally. It sends a 

 bundle of fibres to the coccyx [m. recto-coccygeus]. Below, the longitudinal layer 

 passes between the two sphincters and breaks up into numerous bundles which are 

 interwoven with the external sphincter and levator ani, some of them terminating 

 in the circumanal skin. 



Its mucous membrane is thicker than that of the rest of the large intestine. Certain 

 folds, chiefly longitudinal in direction, are seen in the lax state of the tube, which disappear 

 when distended, but Houston had described three permanent oblique transverse folds [plicae 

 transversales recti] (fig. 936), containing bundles of non-striated muscle-cells, which project 

 into the lumen of the tube: one is on the right at the level of the reflection of the peritoneum 

 from the rectum; and two are on the left, one above and one below the right fold. That upon 

 the right side is the largest and most constant, and its muscular bundle is sometimes called 

 the sphincter tertius. It is located about 7.5 cm. above the anus. These folds, like the corre- 

 sponding semilunar folds of the colon, when well marked involve the entire wall. 



The mucous membrane of the upper portion of the anal canal presents a series of vertical 

 folds knowTi as rectal columns [columnse rectales] (columns of Morgagni), containing bundles 

 of non-striated muscle longitudinally arranged. These columns become more prominent as 

 they extend downward. Just above the anus each two adjacent columns are united by an 

 arch-hke fold of mucous membrane, these folds forming what are known as the anal valves, 

 while the smaU fossse behind them are known as the rectal sinuses. The area below the valves 

 and extending to the anus is termed the annulus hoemorrhoidalis (fig. 936). This is lined by a 

 modified skin, while the area above the valves forms a transition to the typical mucosa of the 

 rectum. 



Minute structure of the large intestine. — In general, the large intestine has the four coats 

 (fig. 939) — mucosa, submucosa, muscularis, and serosa — characteristic of the ahmentary 

 canal. The mucosa lacks the villi and plicse circulares characteristic of the small intestine. 

 It contains many solitary lymphatic nodules, but no Peyer's patches. It differs from the 

 stomach in the absence of foveolse, and m the presence of large numbers of mucous ' goblet 

 cells' found both on the surface and along the numerous crypts of Lieberkuehn (which con- 

 tain no cells of Paneth). The submucosa is much as in the small intestine. The muscularis 

 has a continuous inner circular layer, the outer longitudinal fibres being chiefly gathered into 

 the three bands, the teniae coli, as above mentioned. The serosa is typical, excepting extra- 

 peritoneal areas where the epithehum is lacking. The appendices epiploicae were also mentioned 

 above. 



The caecum and colon present no special features worthy of mention, beyond the typical 

 structure above outUned. 



