THE DIGESTIVE SYSTEM 75 



inches from the pylorus, along with the principal pancreatic duct. 

 In most animals a special reservoir for bile is present in the form 

 of the gall-bladder. The horse has no gall-bladder. Besides 

 having the function of manufacturing bile, the hver acts as a store- 

 house for glycogen until it is required to nourish the body. The 

 liver is also concerned with the production of a large number of 

 enzymes, which dissolve, transform, precipitate, or completely 

 destroy toxic materials in the blood, especially those brought to the 

 liver from the intestines. Wastes are removed from the blood by 

 the hver. The dual functions have compelled us to look upon this 

 gland as both an excretory and secretory organ. 



The pancreas so closely resembles a salivary gland in its struc- 

 ture and physical properties that it is commonly referred to as the 

 abdominal salivary gland. It is situated behind the stomach and 

 largely below the right kidney. There are two ducts : the larger is 

 the pancreatic duct proper, which has a common opening into the 

 small intestine with the bile-duct; the smaller enters on the oppo- 

 site side by a special opening. The function of 'the pancreas is to 

 secrete pancreatic juice. 



The intestinal glands are of two kinds : the glands of Lieber- 

 kiihn are very small, tubular structures, and are distributed 

 throughout the entire intestinal tract; Brunner's glands are found 

 in the first 20 feet of the bowel. Unless a close examination is 

 made these glands are not readily seen, as they are embedded in 

 the intestinal walls. The uses of their secretions will be men- 

 tioned when digestion is studied. 



SECRETION, DIGESTION, AND ABSORPTION 

 Before food is available for use in the body it must be acted 

 upon by various juices. These are supphed by special secretory 

 glands situated along the course of the digestive canal. Three 

 processes are involved in the preparation of food — secretion, diges- 

 tion, and absorption. Secretion is the process of elaborating the 

 juices of the digestive glands (Fig. 22). Digestion is the process 

 of separating the available from the unavailable materials of the 

 food, and consists in dissolving, breaking down, and chemically 

 changing the food so that it can be absorbed by the blood and 

 utihzed by the body. Absorption is the process whereby digested 

 food nutrients are taken up by the venous and lymphatic systems 

 and passed into the general circulation. 



