THE DIGESTIVE SYSTEM. 305 



(3) Gastric juice also acts upon neutral fats, but only on those 

 which are in the form of a fine emulsion, such as yolk of egg or milk. 

 The fats are split into glycerol and fatty acids by the agency of an 

 enzyme, lipase. The fat-splitting function of gastric juice, however, is 

 limited in extent, and is of relatively small importance as compared 

 with that which takes place in the small intestine. Pepsin indirectly 

 assists the digestion of fat by dissolving the cell envelopes of the fat 

 cells of adipose tissue contained in food. In this way fat is set free and 

 prepared for the subsequent digestive action of pancreatic lipase. 



THE SECRETION OF GASTRIC JUICE. 



The mechanism of the secretion of the gastric juice is studied by 

 the subdivision of the stomach in an animal in the manner already 

 described (p. 301), the larger part remaining in continuity with the 

 digestive tract, while the smaller subdivision opens freely on the surface 

 of the body. In such an animal, it has been shown that secretion of 

 juice in the large stomach is accompanied by a proportional secretion 

 of juice in the small stomach, and, further, that the two juices are equal 

 in digestive power. 



The secretion of gastric juice begins about five minutes after an 

 ordinary meal is taken, and continues steadily during the period of 

 digestion of the stomach contents. The initial secretion has been 

 shown to be produced by a nervous mechanism, this being supplemented, 

 after a variable time, usually twenty to forty -five minutes, by a further 

 flow excited by a chemical stimulus. 



The Nervous Mechanism of Gastric Secretion. The normal stimulus 

 which excites the flow of gastric juice is the presence of food in the 

 mouth. If the oesophagus of a dog is divided and the two ends are 

 brought to the surface and fixed there, food may be masticated and 

 swallowed by the animal, but none will reach the stomach. In such a 

 case the food which is eaten all escapes by the oesophageal fistula, and 

 is spoken of as a sham meal. In an animal provided with both 

 oesophageal and gastric fistulse, sham feeding is followed by the secretion 

 of gastric juice, at the same time and in the same way as if the food 

 reached the stomach. This fact points to the probability that the 

 first secretion is due to a nervous reflex, a presumption which is further 

 supported by the observation that, as in the case of the saliva, even the 

 sight or suggestion of food is followed by the appearance of gastric juice. 

 The nervous nature of the stimulus is proved by the effect of division of 

 both vagus nerves to the stomach, after which no secretion takes place on 

 sham feeding or on showing food to the animal. Further proof is 



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