DIGESTION 351 



them, and throw them against its walls. In this way 

 not only are the contents thoroughly mixed, and fresh 

 portions of food constantly brought into contact with the 

 gastric juice secreted mainly in the more passive cardiac 

 end, but a certain amount of mechanical disintegration is 

 brought about ; and this is aided by the digestion of the 

 gelatin-yielding connective tissue which holds together the 

 fibres of muscle and the cells of fat, and the digestible 

 structures in vegetable tissue which enclose starch granules. 

 If milk has formed a portion of the meal, the casein will 

 have been curdled soon after its entrance into the stomach, 

 by the action of the rennet ferment alone when the milk 

 has been taken at the beginning of digestion before the 

 gastric contents have become distinctly acid, by the acid and 

 rennin together when it has been taken later. The casein 

 and other proteids of milk, like the myosin and other 

 proteids of meat, and the globulins, phytovitellins, and 

 other proteids of bread and of vegetable food in general, are 

 acted upon by the pepsin and hydrochloric acid, yielding 

 ultimately peptones ; while variable quantities of these pro- 

 teids and of the acid-albumin and proteoses derived from 

 them may escape this final change, and pass on as such into 

 the duodenum. In the dog, indeed, a meal of flesh has 

 been found to be almost entirely digested to the peptone 

 stage while still in the stomach, leaving little for the pan- 

 creatic juice to do. But we may safely assume that, in the 

 case of a man living on an ordinary mixed diet, much of the 

 food proteids passes through the pylorus chemically un- 

 changed, or having undergone only the first steps of hydra- 

 tion. For, even a few minutes after food has been swallowed, 

 the pyloric sphincter may relax and allow the stomach to 

 propel a portion of its contents into the intestine ; and such 

 relaxations occur at intervals as digestion goes on, although 

 it is not for several hours (three to five) that the greater 

 portion of the food reaches the duodenum. During this 

 period the acidity has at first been constantly increasing, 

 although for a time the hydrochloric acid has combined, 

 as it is formed, with the proteids of the food. The combina- 

 tion, however, does not prevent it from contributing to 



