90 THE PHYSIOLOGY OF DIGESTION AND ABSORPTION 



point to an influence of the central system over secretion. Of 

 course the sympathetic fibers to the vessel walls are indirectly 

 concerned. 



Condition of Food on Entering Stomach. The food 

 enters the stomach in the same condition in which it left the 

 mouth. It has been more or less completely triturated by mas- 

 tication; the whole has been moistened, and a part dissolved 

 by the saliva. All the materials taken in have been thoroughly 

 mixed with each other, and some of the starch has been con- 

 verted into sugar. The reaction is now alkaline, unless the acid- 

 ity of the articles taken has been too great to be overcome by 

 the alkalinity of the saliva in which case there would be no 

 amylolytic change. Excepting starch, all foods entering the 

 stomach are chemically unaffected. It remains to see what hap- 

 pens to the foods under the influence of gastric digestion. These 

 changes are brought about by the gastric juice aided by muscular 

 movements of the stomach. 



Properties and Composition of Gastric Juice. The secre- 

 tion of the glands of the stomach is called gastric juice. Gas- 

 tric juice may be secured in several ways, but the most reliable 

 article for experimentation is taken from a previously established 

 gastric fistula in one of the lower animals. It is a thin, almost 

 colorless liquid of an acid reaction, and a specific gravity of 1005 

 to 1009. Chemically it contains per thousand about 973 parts 

 water and 27 solids. Proteid substances compose some 17 of the 

 27 parts of solid matter. These substances are mainly mucin, 

 pepsin and rennin. The most important non-nitrogenous 

 constituent is free hydrochloric acid. The others are chiefly 

 the chlorides of sodium, potassium, calcium, and ammonium, 

 and the phosphates of iron, calcium, and magnesium. The 

 amount of gastric juice secreted in twenty-four hours is from six 

 to fourteen pounds. Gastric juice will resist putrefaction for a 

 long time, probably on account of the free acid. Its digestive 



