THE LARGE INTESTINE 1179 



called to the transverse folds (of Houston) and the rectal columns, sinuses and valves. Just 

 above the valves, the mucosa is transitional, the epithelium being partly stratified, and the 

 crypts of Lieberkuehn few and scattering. Below the valves, the annulus hsemorrhoidalis is 

 lined by a modified skin. Hairs and sebaceous and sweat glands do not appear until just 

 outside the anal orifice. The thickening of the circular muscle to form the internal sphincter, 

 and the somewhat uniform disposition of the longitudinal muscle have already been mentioned, 

 as well as the absence of a serous coat in the lower portions. 



Blood-vessels. — The large intestine is supplied with blood by the branches of the superior 

 mesenteric and inferior mesenteric arteries, while it also receives a blood-supply from the 

 internal iliac at the rectum. The vessels form a continuous series of arches from the caecum, 

 where the vasa intestini tenuis anastomose with the ileo-colic, the first branch of the superior 

 mesenteric given to the large intestine. 



The blood-supply of the rectum is from the inferior mesenteric by the superior haemorrhoidal, 

 from the hypogastric (internal iliac) by the middle haemorrhoidal, and from the internal pudic 

 by the inferior haemorrhoidal. The vessels at the lower end of the rectum assume a longitudinal 

 direction, communicating freely near the anus, and less freely above. 



The blood of the large intestine is returned into the portal vein by means of the superior 

 mesenteric and inferior mesenteric veins. At the rectum a communication is set up between 

 the systemic and portal system of veins, since some of the blood of that part of the intestine is 

 returned into the hypogastric (internal ihac) veins. In the lower end of the rectum the veins, 

 hke the arteries, are arranged longitudinally. This arrangement is called the haemorrhoidal 

 plexus. 



The vermiform process is supplied by a special branch of the ileo-colic artery (fig. 934). 

 This branch, the appendicular artery, crosses behind the terminal portion of the ileum (where 

 pressure may obstruct the circulation) to enter the mesenteriolum. An accessory artery of 

 small size also descends along the medial margin of the colon and caecum, entering the base of 

 the appendix. 



The nerves and lymphatics of the large intestine differ in no important particular from those 

 of the small intestine, so far as their relations within the intestinal wall are concerned. 



The efferent lymphatic vessels in general follow the blood-vessels and pass through cor- 

 responding lymph nodes in the various regions (see p. 734). Those of the caecum and vermi- 

 form process pass through the appendicular and ileo-caecal nodes; those of the colon through 

 mesocoHc and mesenteric nodes. Those of the descending and sigmoid colons connect with 

 the inferior mesenteric and lumbar nodes. The superior zone of the rectum is drained by 

 lymphatics passing to the ano- rectal and inferior mesenteric nodes; the middle zone (region of 

 rectal columns) to nodes along the three haemorrhoidal arteries; the inferior zone (anal in- 

 tegument) chiefly to the superficial inguinal nodes. 



Development of the large intestine.— At an early stage in the development of the intestinal 

 canal, when this presents a single primary loop and soon after this loop has turned on its axis, 

 there is observed on the left half of the loop, near its top, an enlargement which marks the be- 

 ginning of the large intestine. With further growth this enlargement develops a lateral out- 

 growth on the side opposite to that to which the mesentery is attached, therefore free from the 

 mesentery. A conical projection of the large intestine or colon beyond the place where this is 

 joined to the small intestine is thus formed. This conical projection or pouch of the large in- 

 testine, which continues the colon somewhat beyond the insertion of the small intestine, develops 

 into the caecum and the vermiform process. It does not present, in its further growth, a uniform 

 enlargement. The portion nearest the colon grows in size more rapidly than the terminal por- 

 tion, this difference in size becoming more apparent as development proceeds, the smaller 

 terminal portion forming the vermiform process. On the return of the intestine to the peritoneal 

 cavity (in embryos of about 40 mm.) the caecum lies on the right side, immediately below the 

 hver. During the later foetal months the caecum gradually descends into the right iliac fossa, 

 and there is thus estabhshed an ascending colon. The caecum may, however, even in the adult, 

 retain its embryonic position on the right side immediately beneath the Uver, or may descend 

 farther than usual. 



The ascending and descending colons, the sigmoid meso-colon (in part), and the rectum 

 with corresponding portions of the mesorectum, become adherent to the posterior body wall 

 during the fourth and fifth foetal months. At the same time, the posterior layer of the great 

 omentum becomes fused with the upper (anterior) surface of the transverse meso-colon. The 

 layer of retroperitoneal fascia corresponding to the obliterated mesocolon is shown in fig. 1005. 

 Variations in the process of fusion give rise to numerous peritoneal variations in the adult. 



The sigmoid colon is relatively long at birth. On account of the relatively small size of the 

 true pelvic cavity, both sigmoid colon and coils of ileum are usually excluded from it in the 

 foetus and infant. 



In foetuses of four to six months (length 100 mm. to 240 mm.) transitory viUi appear in 

 the mucosa throughout the large intestine, including the vermiform process. They appear 

 in rows, corresponding to longitudinal folds. Their early obhteration is possibly due to dis- 

 tention of the gut by the meconium. The glands bud off hke those of the small intestine. 

 Lymphoid nodules are present abundantly in the vermiform process at birth (Johnson). The 

 circular muscular layer begins to appear in the lower part of the large intestine at 23 mm.; 

 the teniae at 75 to 99 mm. (F. T. Lewis). 



Development of the rectum and anus. — The posterior end of the primitive intestine or arch- 

 enteron, designated the hind-gut, presents a terminal portion which is somewhat dilated and 

 known as the cloaca, into the lateral and ventral portions of which open the Wolffian ducts, 

 and from the ventral portion of which arises the allantois. The ventral portion of the cloaca, 

 which is an entodermal structure, comes in contact with the ectoderm to form the cloacal 

 membrane, and this forms the floor of a slight depression. For a time the cloaca or hind-gut 

 extends for some distance caudal to the cloacal membrane, forming what is known as the post 

 anal gut; this, however, soon disappears. Early in the development of the human embryo 



