LARGE INTESTINE. 217 



maining layers resemble those of the small intestines. The outer longitudi- 

 nal layer of the muscularis is thin except where its fibers are gathered into 

 three longitudinal bands or tacniae, nearly equidistant from one another, 

 which terminate in the corresponding layer of the vermiform process. 

 The latter is uniform and not separated into taeniae. Since the longi- 

 tudinal bands are shorter than the inner layers of the colon, internal trans- 

 verse folds are produced, called plicae semilunares. Inasmuch as the 

 circular muscle layer is included in them, they differ from the plicae cir- 

 culares of the small intestine. They occur at considerable intervals and 

 between two successive plicae the wall of the colon exhibits a saccular 

 dilatation or haustrum. The valve of the colon (valvula coli) is a pair of 

 folds or labia, which are similar in structure to the semilunar folds; that is, 

 they include fibers of the circular muscle layer, but the shorter layer of 

 longitudinal fibers passes directly from the ileum to the colon without 

 entering the valves. Figures of the bands, folds, pouches, and valves of the 

 colon may be found in the text-books of anatomy, and to these the stu- 

 dent should refer. The serosa of the colon contains lobules of fat which 

 form pendulous projections known as appendices epiploicae. 



RECTUM AND ANUS. 



The rectum agrees in its general structure with the colon, and has even 

 longer glands (0.7 mm.). Its linings present transverse folds (plicae trans- 

 versales) and in the anal region there are several longitudinal folds, the 

 rectal columns. In this region the musculature is highly developed. The 

 muscularis mucosse becomes thicker and enters the columns. The cir- 

 cular layer of the muscularis terminates as a special accumulation of fibers, 

 the internal sphincter of the anus. Just beyond it is the external sphincter, 

 a striated muscle of the perineal group. The three taeniae of the colon 

 unite so as to form two in the rectum, a ventral [anterior] and a dorsal 

 [posterior], but by the development of fibers between them the longitudinal 

 layer becomes essentially complete and uniform. It terminates by joining 

 the internal sphincter and neighboring muscles, and by ending in the sub- 

 epithelial tissue. 



A short distance within the internal sphincter the simple columnar 

 epithelium abruptly becomes a thick stratified layer with flat outer cells. 

 Its base rests upon vascular papillae. The rectal glands extend for a 

 short distance into the region of stratified epithelium. The circumanal 

 glands which appear as modified sweat glands occur beyond the anus, in 

 the skin. 



The vessels and nerves of the large intestine are distributed essentially 

 as in the small intestine, except for the absence of villi. The great abun- 



