CHAP. XV.] ALIMENTATION. 171 



During the processes of mastication, insalivation, and deglu- 

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

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

 sugar ; thirdly, it acquires a more or less alkaline reaction. 1 



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, 

 grajdsh 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, colourless, 

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

 and is dependent for its specific action upon a ferment called 

 pepsin, which 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 

 envelopes of the fat-cell and set the fat free, but it has no 

 power to emulsify them. The essential property of gastric 

 juice is the power it has of dissolving proteid matters and of 

 converting them into a substance called peptone. Whatever 

 the proteid may be, whether the albumin of eggs, the gluten of 

 flour in bread, the myosin in flesh, the result is the same, pepsin, 

 in conjunction with an acid at the temperature of the body, 

 transforms them into peptones. 



Peptones readily dissolve in water, and pass with ease through 

 animal membranes. They are probably absorbed, as soon as 

 1 Weak alkalies stimulate the gastric glands to more abundant secretion. 



