233 GASTRIC DIGESTION. 



contractions of the stomach upon the food contained in its cavity is to sub- 

 ject it to a tolerably uniform pressure in the cardiac portion, the general 

 tendency of the movement being toward the pylorus, along the 'greater curva- 

 ture, and back from the pylorus toward the great pouch, along the lesser 

 curvature. At the constricted part which separates the cardiac from the 

 pyloric portion, there is an obstruction to the passage oi the food until it has 

 been sufficiently acted upon by the secretions in the cardiac division to have 

 become reduced to a pultaceous consistence. The alimentary mass then 

 passes into the pyloric division, and by a more powerful contraction than 

 occurs in other parts of the stomach, it is passed into the small intestine. 



The revolutions of the alimentary mass, thus accomplished, take place 

 slowly, by gentle and persistent contractions of the muscular coat ; the food 

 occupying two or three minutes in its passage entirely around the stomach. 

 Every time that a revolution is accomplished, the contents of the stomach 

 are somewhat diminished in quantity ; probably, in a slight degree, from ab- 

 sorption of digested matter by the stomach itself, but chiefly by the gradual 

 passage of the softened and disintegrated mass into the small intestine. This 

 process continues until the stomach is emptied, lasting between two and four 

 hours ; after which, the movements of the stomach cease until food is again 

 introduced. 



Regurgitation of food by contractions of the muscular coats of the stom- 

 ach, eructation, or the expulsion of gas, and vomiting are not physiological 

 acts. It has been shown that vomiting is produced by contractions of the 

 abdominal muscles and the diaphragm, compressing the stomach, which is 

 passive, except that the pyloric opening is firmly closed, the cardiac opening 

 being relaxed. Eructation, although usually involuntary, is sometimes under 

 the control of the will. When it occurs, while it is difficult or impossible to 

 prevent the discharge of the gas, the accompanying sound may be readily 

 suppressed. Eructation frequently becomes a habit, which in many persons 

 is so developed by practice that the act may be performed voluntarily at any 

 time. The gaseous contents of the stomach during digestion are composed 

 of oxygen, carbon dioxide, hydrogen and nitrogen, in proportions that are 

 very variable. 



