DIGESTION 201 



glands embedded in its thick walls or else there are larger 

 glands lying outside the stomach wall, but connected with it by 

 ducts. From the stomach the digestive tract continues, first 

 as a slender "small intestine" which farther on expands into a 

 wider " large intestine." The latter ends at the vent. 



460. Quite generally the digestive tract of Vertebrates is 

 differentiated into the following series of parts; buccal cavity, 

 oesophagus, stomach, small intestine, and large intestine. Its 

 walls are very muscular, especially those of the stomach and 

 small intestine. The internal surface area is greatly increased 

 by folds and countless minute thread-like elevations, the villi. 

 The teeth with which the mouth is usually armed serve either 

 for seizing and swallowing the prey or for mastication. 



461. The digestive glands are numerous and large. In man 

 there are three pairs of large salivary glands, besides a number 

 of smaller ones, which open into the buccal cavity. But for 

 Vertebrates the general statement also applies, that salivary 

 glands are characteristic only of terrestrial forms. Besides 

 moistening the food for swallowing it the saliva also sometimes 

 seems to soften it preparatory to digestion (birds). In herbiv- 

 orous animals the saliva often contains an amylolytic ferment, 

 ptyalin. Embedded in the thick walls of the stomach are 

 numerous small tubular glands which secrete gastric fluid. 

 This contains hydrochloric acid and a proteolytic ferment, 

 pepsin. 



462. A gland of considerable size, the pancreas, opens into 

 the intestine near the stomach. Its secretion contains three 

 ferments, one, amylopsin, is amylolytic; another, trypsin, is 

 proteolytic, and a third, steapsin, decomposes fats into glycerine 

 and fatty acids. There are also numerous small glands em- 

 bedded in the wall of the intestine, which are said to secrete the 

 ferment, invertin, which is found in the intestine and which 

 inverts maltose into glucose. 



463 . Amylolytic ferments are not all alike. That is, there are 



