The Digestive System, 135 



animals such as the rabbit, the caecum attains an enormous size, hav- 

 ing a capacity nearly equal to that of the rest of the intestinal tract. 

 In these animals, it possibly has an important storage function. 



The greater portion of the large intestine is known as the colon. 

 This consists of three regions : the ascending, transverse, and descend- 

 ing colons. These are actually indistinguishable from one another ex- 

 cept from the position that they occupy in the coelom. Externally the 

 walls of this structure have a pouched appearance ; internally these 

 pouches are seen to form side compartments, slightly separated from 

 the main lumen. They effectively increase the absorptive surface of 

 the large intestine. This pouched appearance is due to the presence 

 of longitudinal muscle fibers which are in the outer surface layer of 

 the large intestine. These fibers are shorter than the total length of 

 the colon. 



Continuous with the descending colon is the rectum. It is about 

 five inches long in the human being and merges into the short anal 

 canal. The posterior opening of the intestinal tract, the anus, is guarded 

 by two sphincter muscles, the internal and external ones. 



Histology. — Fundamentally the layers of tissues forming the walls 

 of the digestive tract are the same throughout its length; however, they 

 are subject to considerable variation in details. 



Starting from the lumen, the layers are as follows : the mucosa 

 which includes the internal glandular epithelium and the connective 

 tissues directly beneath it ; the submucosa which consists mainly of con- 

 nective tissues and contained nerves and blood vessels ; the muscle 

 layers which usually include an inner layer of circular muscles and an 

 outer layer of longitudinal ones ; and the serous layer which is the 

 visceral peritoneum. 



The mucosa is the most variable of the layers. In the esophagus 

 the epithelium is formed of stratified squamous cells. The mucous 

 glands are located in the submucosa and have small ducts leading 

 through the mucosa to the lumen. The epithelium of the stomach 

 mucosa is formed of simple columnar epithelial cells. When the stom- 

 ach is not distended with food, folds are visible over its surface. Tiny 

 shallow pits dot the whole of the mucosa. It is into these pits that 

 the gastric glands discharge their products. In the small intestine, the 

 greatest modification of the mucosa is seen. Like the stomach, the 

 epithelium is of the tall columnar type, but the whole surface is thrown 

 into numerous circular folds. On and between these folds are the 



