98 ANIMAL PHYSIOLOGY. 



time the contact of the acid gastric juice, and have a much 

 larger quantity of its starch acted on in the stomach. The 

 conversion of starch into sugar, however, is neither the only 

 nor even the principal use of the saliva, as is proved, first, 

 from the salivary glands being well developed in carnivorous 

 animals, which, in the natural state, use no starch in their 

 food; and, secondly, from the amount of saliva mixed with 

 the food bearing a direct proportion to its dryness a circum- 

 stance which shows that it is important as a moistener. 



66. In the stomach the food causes a flow of gastric juice, 

 an acid secretion with a peculiar nitrogenous principle, called 

 pepsin. Various acids may be developed among the contents 

 of the stomach, the most important of which are lactic, 

 acetic, and butyric ; but these are generated accidentally in 

 the processes of change, and it has been shown by investiga- 

 tions, both on the lower animals and on man, that free 

 hydrochloric acid is that which is secreted by the glands of 

 the stomach. The use of the acid, however, appears to be 

 to aid the pepsin ; the peculiar properties of that substance 

 being exhibited in none but acid solutions : and for this 

 purpose any acid suffices. Pepsin is obtainable from the 

 mucous membrane of the stomach of animals by the action 

 of water. It is the active principle in the rennet iised in 

 manufacturing cheese; rennet being a preparation of the 

 mucous membrane of a calf's stomach, used on account of the 

 property, which pepsin possesses, of curdling casein. In an 

 acid solution, at a temperature of about 100.F., pepsin dis- 

 solves coagulated albumen, and renders it incapable of being 

 coagulated again by heat. It acts in like manner on nearly 

 all albuminoids, and the substances into which it converts 

 them are called peptones; but the chemical nature of these 

 is not thoroughly understood. It likewise dissolves gelatin 

 and gelatiniferous tissue, converting them into a solution 

 which does not jelly on cooling ; but it has no action on 

 either oil or starch. The gastric juice therefore digests none 

 but nitrogenous substances. 



67. The mucous membrane of the stomach, by which the 

 gastric juice is secreted, is thick, soft, and smooth. Looked 

 at under the microscope, the surface is seen to be thrown, 

 into shallow pits, into each of which open several small 



