94 HUMAN BIOLOGY 



(over 99 per cent), free hydrochloric acid and a digestive fer- 

 ment' called pepsin. 



128. Muscles of the stomach. — The chief function of the 

 human stomach is to secrete the gastric juice and to mix this 

 juice thoroughly with the food. The muscular walls are 

 well adapted for this piu'pose. When the food reaches the 

 stomach, the gastric juice oozes out upon it, and the mixture 

 is pushed back and forth and up and down by the successive 

 action of the different layers of muscles. The return of the 

 food to the mouth cavity is prevented by the contraction of 

 the circular muscles at the lower end of the gullet, except 

 in the case of nausea, when they relax and allow the stomach 

 to rid itself of its contents. The circular muscle at the pyloric 

 end of the stomach (Fig. 26) relaxes from time to time, and 

 the partially digested food is pushed on into the intestine. 



Fortunately for the well-being of the body, all these pro- 

 cesses are entirely automatic; that is, they are carried on 

 without our conscious direction. The muscles of the ali- 

 mentary canal for this reason are called involuntary (Latin, 

 in = without + voluntas = will). 



129. Digestion in the stomach. — - The gastric juice has 

 practically no effect on the nutrients starch and fat. The 

 saliva, however, that is mixed with the food and swallowed 

 with it continues to act upon the starch for a time, particu- 

 larly in the upper part of the stomach. Sugars and soluble 

 salts (that is, salts that dissolve in water), if not dissolved in 

 the mouth, are readily liquefied by the water of the gastric 

 juice. Certain mineral food substances, however, like phos- 

 phate of lime found in milk, are not soluble in water, and these 

 insoluble salts reach the stomach unchanged. The following 

 experiment illustrates the way in which mineral matters 

 are made hquid by the hydrochloric acid in the gastric juice. 



