THE CHEMISTRY OF THE ANIMAL BODY 159 



0*6 per cent., while the last usually has about 4 or 5 per cent, 

 of sulphur. Keratin is the main constituent of horns, hoofs, 

 skin, feathers, hair, wool, nails, (kc. It is insoluble in water, 

 alcohol, or ether, but by heating with water under pressure to 

 150° or 200° 0. it can be rendered soluble, and then constitutes 

 glue. 



Digestion. — An important process by which the food of an 

 animal is rendered capable of being absorbed into the system 

 and utilised in building up or renewing the tissues of the body. 

 It is accomplished partly by mechanical means, but mainly by 

 chemical changes, which are produced chiefly by the action of 

 enzymes. 



The first step is mastication, by which the food is subdivided 

 and crushed by the action of the teeth and thoroughly mixed 

 with salivaf a special secretion poured by glands into the 

 mouth. Saliva is a highly dilute liquid of faint alkaline re- 

 action, and contains an enzyme, pti/alin, or salivary diastase, 

 which has the power of bringing about the same changes as 

 are produced by plant diastase, viz., the conversion of starch 

 into sugar (maltose). Ruminants, whose food usually con- 

 tains much starchy material, secrete enormous quantities of 

 saliva — estimated in the case of the ox at about 1 cwt. per day. 



The food, after mastication, passes into the stomach, though 

 in the case of ruminants it is brought back from the paunch 

 or rumen into the mouth to undergo a second mastication 

 (" chewing the cud "). It then passes into the stomach, where 

 it meets with the characteristic secretion — the gastric juice* 

 The gastric juice contains various salts (chlorides and phos- 

 phates of calcium, magnesium, sodium, and potassium), free 

 hydrochloric acid, and two enzymes, pepsin and rennet (or 

 chymosin). The former has the power of converting insoluble 

 proteids into soluble and diffusible albumoses and peptones, 

 the latter of coagulating casein. These properties are pos- 

 sessed in acid, but not in alkaline solutions. Pepsin acts best 

 in a liquid containing from 0*1 to 0*3 per cent, of free hydro- 



h 



