THE DIGESTION OF FOOD 103 



the food passes in thin sheets, an arrangement which 

 seems to have for its purpose the further grinding of the 

 food so that when it finally reaches the fourth and last 

 compartment it is in a very finely divided condition and 

 is thoroughly prepared for the action of the juices that 

 are subsequently poured upon it. 



143. The abomasum. It is at the last stage of the 

 journey of the food through this complicated stomach 

 that it is submitted to the true gastric digestion. As a 

 matter of fact, the abomasum, or rennet, is regarded as the 

 true stomach, the other three sacs being considered as 

 enlargements of the esophagus. In the calf, the rennet is 

 only partly developed, the other divisions not coming into 

 use until the animal takes coarse foods in considerable 

 quantity. The fourth stomach is larger than either the 

 second or third. It receives directly from the omasum 

 the finely divided food, upon which it pours the gastric 

 juice, a liquid that is secreted in large quantity by glands 

 located in its inner or mucous membrane. 



144. The gastric juice. This juice, like all the diges- 

 tive fluids, is mostly water, the proportion being between 

 ninety-eight and ninety-nine parts to less than two parts 

 of* various compounds. The latter consist of ferments, a 

 certain amount of free hydrochloric acid and a variety of 

 mineral compounds, prominent among which are calcium 

 and magnesium phosphates and the chlorides of the 

 alkalies, sodium chloride being especially abundant. 



145. Artificial digestion. Especial interest pertains 

 to the ferments of the gastric juice, one of which, in con- 

 nection with free hydrochloric acid, causes a most impor- 

 tant change in the proteins of the food by reducing them 

 through hydrolytic and other cleavages to soluble forms. 

 We know quite definitely about this action, because it can 



