vii THE DIGESTIVE SYSTEM AND ITS FUNCTIONS 151 



the folds lose their regular arrangement, and in the posterior 

 third they assume a longitudinal direction. 



The large intestine is composed of the same layers as the 

 small. The inner surface is thrown into folds, which at the 

 proximal end form an irregular network, but in the rectum 

 they become longitudinal. The epithelium of the mucosa 

 consists of cylindrical cells, among which numerous goblet 

 cells are to be found. 



The Pancreas. — The pancreas is an elongated gland of 

 irregular shape situated between the stomach and the duo- 

 denum, and extending from the liver to within a short dis- 

 tance of the pylorus. It is traversed by the common bile 

 duct into which its ducts enter. Of these there is a princi- 

 pal duct, and several smaller ducts from the portion of the 

 gland near the liver. 



The pancreas is a much-branched tubular gland, the ter- 

 minal branches of the glands being often curved and twisted 

 in an irregular manner. The tubules are coated externally 

 with a basement membrane, and held together by a delicate 

 connective tissue in which lie the blood vessels and nerves. 



The secretory cells of the tubules contain numerous 

 zymogen granules, which, when the frog is in a hungry state, 

 are found in great abundance, especially at the inner or free 

 end of the cell. These disappear after the animal is fed, 

 like the granules in the glands of the stomach. A peculiar 

 darkly staining body (paranucleus, nebenkern) is usually 

 found near the nucleus toward the outer or basal end of 

 the cell. 



The fluid secreted by the pancreas is alkaline, mainly 

 from the presence of sodium carbonate (Na 2 C0 3 ), and it 

 contains three ferments : steapsin, which causes a splitting of 

 fats into fatty acid and glycerin ; amylopsin, which converts 

 starch into sugar ; and trypsin, which converts proteids into 



