CHAP. XVI.] ALIMENTATION. 193 



effected at the temperature of the body, in a slightly alkaline 

 solution, saliva that is distinctly acid hindering or arresting 

 the process. Boiled starch is changed more rapidly and com- 

 pletely than raw, but the food is never retained in the mouth 

 long enough for the saliva to more than begin the transforma- 

 tion of starchy matters. After leaving the mouth, further con- 

 version of starch into sugar is arrested by the acid reaction of 

 the gastric juice, and digestion of this class of food-stuffs is 

 practically suspended until they again come in contact with 

 the alkaline secretions in the upper part of the small intestine. 



During the processes of mastication, insalivation, and deglu- 

 tition, the food is first reduced to a soft pulpy condition ; sec- 

 ondly, any starch it may contain begins to be changed into 

 sugar ; thirdly, it acquires a more or less alkaline reaction. 



Changes the food undergoes in the stomach. The entrance of 

 food into the stomach acts as a stimulant to the whole organ. 

 The blood-vessels dilate, the glands pour out an abundant secre- 

 tion upon the mucous lining, and the different layers of the 

 muscular coat are excited to a continuous action. Delayed in 

 the stomach by the contraction of the pyloric ring-muscle, the 

 pulpy mass of food is carried round and round, and thoroughly 

 mixed with the gastric juice until it is dissolved into a thick, 

 grayish soup-like liquid, called chyme. The chyme thus formed 

 is from time to time ejected through the pylorus, accompanied 

 by morsels of solid, less well-digested matter. This ejection 

 may occur within a few minutes after the entrance of food into 

 the stomach, but does not usually begin until from one to two 

 hours after, and lasts from four to five, at the end of which 

 time the stomach is, after an ordinary meal, completely emptied. 



Gastric juice. Gastric juice, secreted by the small, tubular 

 glands in the mucous lining of the stomach, is a thin, colour- 

 less, or pale yellow fluid, of an acid reaction. It contains few 

 solids, and is dependent for its specific action upon two enzymes 

 called pepsin and rennin. Pepsin is only properly active in an 

 acid solution, and we therefore find that free hydrochloric acid 

 in the proportion of 0.2 per cent is always present in normal 

 gastric juice. 



Action of gastric juice upon the food. The gastric juice has 

 no action upon starch, and upon fats it has at most a limited 

 action; that is, if adipose tissue be eaten, it will dissolve the 



