DIGESTION AND RESORPTION 79 



2. The invertases, sucrase, maltase and lactase (in the in- 

 testinal juice), acting upon di-saccharids. 



3. The proteases, pepsin (in the gastric juice), trypsin (in 

 the pancreatic juice) and erepsin (in the intestinal juice), act- 

 ing upon proteins. 



4. The lipase, steapsin (in the pancreatic juice), acting upon 

 the lipoids, or specifically the fats. 



115. The mouth. The mouth is the organ of prehension 

 and mastication. Chiefly by means of the tongue, lips and 

 teeth, feed is seized and introduced into the digestive cavity, 

 while the teeth serve also to grind it up, rendering it capable 

 of being swallowed and also exposing more surface to the action 

 of the digestive fluids. The mouth also receives the secretion 

 of three pairs of glands called the salivary glands whose product 

 is known as the saliva. These three pairs are called, respec- 

 tively, the parotid, the submaxillary and the sublingual glands. 



The mixed saliva, consisting of the secretion of all three pairs 

 of salivary glands, together with the comparatively insignificant 

 amounts secreted by the various smaller glands of the mouth, 

 is a thin, colorless, watery, slightly viscid liquid .of alkaline 

 reaction. The organic matter of the saliva includes a trace of 

 albumin, more or less mucus, and the enzym ptyalin, 1 which 

 is its active constituent. 



The saliva has both physical and chemical functions. The 

 presence of feed in the mouth, its taste, odor or sometimes even 

 sight, causes active secretion of saliva, which is mixed with the 

 feed in the act of mastication and moistens and lubricates it 

 so that it can be swallowed. With dry feeding stuffs, the 

 amount of saliva required for this purpose is very large. The 

 total secretion has been estimated at about 84 pounds per day for 

 the horse and at least 112 pounds per day for the ox, although 

 varying greatly with the dryness of the feed. Besides moisten- 

 ing and lubricating the feed, the saliva has also a chemical action 

 upon it. In a slightly alkaline medium and at body tempera- 

 ture, the ptyalin acts upon the starch of the feed, converting 

 it ultimately into maltose. 



116. The stomach. From the mouth, the feed in the act 

 of swallowing passes through the esophagus, or gullet, to the 

 stomach which, except in fowls, is the first enlargement of the 



1 Not present in the saliva of carnivora. 



