LARGE INTESTINE 



273 



of Paneth are absent. Striated cuticular borders appear near the out- 

 lets of the glands, and are well developed upon the columnar cells lining 

 the intestinal lumen. Solitary nodules are numerous, especially in the 

 caecum. They may extend through the muscularis mucosae and expand 

 in a flask-shaped manner in the submucosa (Fig. 268) ; in peripheral sections 

 of such a nodule the stalk by which it joins the tunica propria may not be 

 included, and the area of lymphoid tissue may seem to be wholly in the 

 submucosa. The latter is a connective tissue layer like that of -the small 

 intestine. 



The tunica muscularis of the colon and caecum has a characteristic 

 arrangement not found in the vermiform process. The longitudinal 

 smooth muscle fibers of the outer layer be- 

 come gathered into three equidistant longi- 

 tudinal bands or t<zni<z (Fig. 269); between 

 them the longitudinal fibers form a thin layer 

 which may be interrupted. The taeniae come 

 together at the root of the vermiform process 

 and are continuous with its outer muscle 

 layer. Since the longitudinal muscle layer 

 does not elongate as rapidly as the parts 

 within it, the inner layer of circular smooth 

 muscle, together with the mucosa and sub- 

 mucosa, become thrown into a succession of 

 transverse crescentic folds or plica semi- _ 



FIG. 269. VERMIFORM PROCESS (V. p.), 



lunares. The horns of the crescents are op- lu M M (no 6 . AN N D D COLON (Co H ) E 

 posite the taeniae. Between the semilunar (Aft f s botta -) 



h., Haustra; t., tsenia. 



folds the wall of the large intestine bulges 



outward, forming the haustra (Lat., buckets) as shown in Fig. 269. The 

 valve of the colon (valvula coli} is a pair of folds or labia, which resemble 

 the semilunar folds; that is, they include fibers of the circular muscle layer, 

 but the layer of longitudinal fibers passes directly from theileum to the colon 

 without entering the valves. The serosa of the colon contains lobules of 

 fat which form pendulous projections known as appendices epiploica. 



RECTUM. 



The rectum is divided into two parts, an upper which extends from the 

 third sacral vertebra to the pelvic diaphragm, and a lower which continues 

 downward to the anus. The lining of the first part is thrown into several 

 folds, the plica transfer sales recti (valves of Houston). These are large 

 semilunar folds which usually extend only part way around the rectum, 

 but they have been described in some cases as having a spiral arrange- 

 ment. The second part of the rectum, the pars analis recti (anal canal), 

 18 



