9O THE PHYSIOLOGY OF DIGESTION AND ABSORPTION 



such fibers' must exist in the vagifc. Section of it (and the 

 sympathetic), however, does not entirely stop the secretion, 

 but incidents referred to in a preceding section, such as se- 

 cretion at sight of food, or when food is chewed and not 

 swallowed, certainly point to an influence of the central sys- 

 tem over secretion. Of course the sympathetic fibers to the 

 vessel walls are indirectly concerned. 



Condition of Food on Entering Stomach. The food en- 

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

 mouth. It has been more or less completely triturated by 

 mastication ; the whole has been moistened, and a part dis- 

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

 thoroughly mixed with each other, and some of the starch 

 has been converted into sugar. The reaction is now alka- 

 line, unless the acidity 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 happens to the foods under the influence 

 of gastric digestion. These changes are brought about by 

 the gastric juice aided by muscular movements of the stom- 

 ach. 



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 reli- 

 able 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 spe- 

 cific gravity of 1005 to 1009. Chemically it contains per 

 thousand about 973 parts water and 27 solids. Proteid sub- 

 stances 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 hydro- 

 chloric 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 



