The Animal Body — Digestion— Metabolism. 21 



Pepsin acts only in Meak acid solutions, converting the very com- 

 plex proteins into soluble and simpler, tlio still complex, products 

 known as proteoses and peptones. Proteoses and peptones are solu- 

 ble nitrogenous compounds, simpler than the proteins from which 

 they originate. They are the result of the partial cleavage of pro- 

 loins with the addition of water. 



Rennin is the enzyme which curdles milk. The membranous lin- 

 ing of the stomachs of calves yields the rennet of commerce, which 

 contains this enzyme. One part of rennin will coagulate 400,000 

 parts of milk. This enzyme is an interesting provision of nature 

 for conserving milk so the animal may get the full value from it. 

 A\X\io liquid, milk is not in condition to be taken directly into the 

 animal system, but, like solid foods, must first undergo digestion. 

 ]\Iilk being liquid, the stomach would naturally pass it quickly on 

 to the small intestine, and if this occurred it would not be suffi- 

 ciently acted on by the pepsin. Rennin quickly converts the milk 

 into a solid curd which is easily retained by the stomach until dis- 

 solved by the action of the digestive juice. 



Acid destroys the power of ptyalin to convert starch into sugar. 

 The construction of the stomach, however, is such that the action 

 of ptyalin on the food after it reaches that organ, following masti- 

 cation, is not too promptly checked. The first portion of the stom- 

 ach, into which the gullet directly leads, secretes pepsin but no 

 acid. The action of ptyalin on the starches of the foods continues, 

 therefore, in this part of the stomach. The intestinal or rear end of 

 the stomach, on the other hand, secretes little pepsin but much hy- 

 drochloric acid. Here the conversion of the starches into malt 

 sugar by the pytalin ceases, and pepsin digestion becomes active. 

 Only the preliminary steps of digestion are accomplished in the 

 stomach, and relatively little absorption of the digested nutrients 

 takes place from it. Sugars may be absorbed to some extent, but 

 the proteoses and peptones produced from the breaking up of pro- 

 tein, and also the fats, are mostly carried into the small intestine 

 along with the other matter. 



Soon after the food reaches the stomach that organ begins a series 

 of orderly movements for the delivery of its contents into the small 

 intestine. In this delivery the stomach contracts at the middle re- 

 gion, and the wave of contraction proceeds slowly and regularly to- 

 ward the intestinal end, one wave following another. Every time 

 the contraction reaches the rear end of the stomach, the ring of 

 muscles which keeps the stomach shut off from the small intestin.^ 



