112 



THE HUMAN MECHANISM 



in amount, this gastric juice is thoroughly mixed with it; 

 when, however, the food is more or less solid and bulky, only 

 the outer layers, which are in immediate contact with the 

 walls of the stomach, are mixed with the juice. At least this 



is true at the cardiac end ; the 

 cavity of the pyloric portion is so 

 small and the amount of move- 

 ment there so great that all por- 

 tions of the pyloric contents are 

 thoroughly mixed with gastric 

 juice ; in the much larger cardiac 

 portion the central mass of the 

 food may receive no gastric juice 

 and thus remain, for an hour or 

 more after the meal, neutral or 

 alkaline in reaction. Under these 

 circumstances very considerable 

 amounts of starch may continue 

 to undergo the salivary digestion 

 begun in the mouth. 



Any chemical action is aided by 

 agitation, since the reacting com- 

 pounds are thus brought into more 

 intimate union ; and observation 

 of the working stomach shows 

 that while the cardiac portion 

 makes no movements, but merely 

 keeps up a steady contraction 

 and thereby exerts a moderate 

 pressure upon its contents, the pyloric portion executes, from 

 a very early stage of digestion and throughout the whole proc- 

 ess, a series of contractions which gradually bring about a 

 thorough mixture of the contents and rub down the softened 

 food into smaller and smaller masses. These contractions 

 consist of rings of constriction which arise at the beginning 



FIG. 56. Outline of the contents 



of the stomach of a cat at three 



stages of the digestion of a meal 



taken about 11 A.M. 



Showing the peristaltic constric- 

 tions which pass over the pyloric 

 portion and the diminution of the 

 quantity of food in the cardiac 

 end. (Full description given in 

 sect. 22) 



