S DIGESTION 



derives from the parotid. Its mucin is derived from the 

 submaxillary, and its ferment chiefly from the same 

 source. Its turbidity is due to salivary corpuscles, and 

 epithelial elements. The salivary secretion of the dog 

 (submaxillary) contains no ferment, and no sulphocyanate. 

 It yields a very large proportion of CO 2 to the mercurial 

 vacuum. 



Saliva owes its value in digestion chiefly to its diastatic 

 ferment, but it is also of use as a solvent and lubricant. 



Digestion in tJie Stomach. 



The relative importance of gastric as compared with 

 salivary digestion varies in different classes of animals 

 according to the nature of their food. In the carnivora 

 the food enters the stomach unchanged, and remains there 

 for many hours. The stomach is of relatively large size, 

 and its whole surface is provided with peptic glands. In 

 the herbivora, as, e.g., in the horse, the thoroughly masti- 

 cated and insalivated food is retained only for a very short 

 time in the stomach. The organ is accordingly very 

 small, and only a part of its mucous lining is digestive. 



Human gastric juice is a colourless transparent liquid 

 of very low specific gravity (1005). It contains neither 

 albumin nor mucin, and may be regarded as a solution of 

 pepsin, hydrochloric acid, chloride of sodium, and other 

 salts. It is secreted by the peptic cells of the glands of 

 which the digestive part of the mucous membrane chiefly 

 consists. The secretion takes place in answer to mechan- 

 ical or chemical stimulation of the mucous surface ; the 

 act is attended with increased circulation of blood in the 

 mucous membrane. 



The process of gastric digestion consists in the trans- 

 formation of the albuminous bodies of the food into acid- 

 albumin and peptone (parapeptone), under the combined 

 influence of pepsin and of a free acid. In the dog, the 



