268 THE ANIMAL. XII. 



These changes are brought about by the process of digestion, 

 which consists mainly in breaking down insoluble, complex 

 carbon compounds into simpler soluble substances, chiefly by 

 the aid of enzymes or unorganised ferments. Digestion is 

 accomplished partly by mechanical, partly by chemical means. 



The process commences with mastication the food is sub- 

 mitted to a comminuting action by the teeth and tongue and 

 at the same time is mixed with saliva. This is a very dilute 

 solution of faint alkaline reaction, containing various sub- 

 stances, secreted by special glands and poured into the mouth. 

 It has been estimated that an ox secretes 112R>. of saliva daily. 

 The results of investigations as to the composition of saliva 

 show it to contain only from '4 to 1 % of solid matter. In 

 human saliva, potassium thiocyanate is usually present, its 

 average amount being probably about -01 %. 



Alkaline chlorides, phosphates, and sulphates are also 

 present, together with mucin, a proteid body of slimy con- 

 sistency. Of special importance is the characteristic enzyme 

 of the saliva ptyalin or salivary diastase. This enzyme, 

 which works most rapidly at about 30 and is destroyed at about 

 65 or 70, has powers similar to those of plant diastase, i.e., it 

 converts starch, first into soluble starch, next into dextrin, and 

 finally into maltose. The conversion of the starch of food into 

 sugar is commenced, but not completed, in the mouth. 



The food next passes into the stomach, which in ruminating 

 animals is divided into several parts and from which the food 

 can be brought back into the mouth to undergo further masti- 

 cation ("chewing the cud"). In the stomach gastric juice is 

 brought into contact with the food. This is a colourless or 

 slightly yellow liquid secreted in the stomach, containing free 

 hydrochloric acid and chlorides and phosphates of calcium, 

 magnesium, and the alkalies. Its characteristic constituents, 

 however, are the enzymes, pepsin, which has the power of 

 converting proteids into albumoses and peptones, thereby ren- 

 dering them soluble and diffusible, and rennet or chymosin, 

 which coagulates casein. These enzymes have not been 

 obtained in a pure state and to some extent probably exist as 

 " zymogens," i.e., substances which yield the true enzymes on 

 treatment with an acid. 



