22 FEEDS AND FEEDING 



as proteoses and peptones. Proteases and peptones are soluble nitroge- 

 nous compounds, simpler than the proteins from which they originate. 

 They are the result of the partial cleavage of proteins with the addition 

 of water. ' 



Rennin, the other enzyme of the gastric juice, changes milk into a solid 

 curd. This enzyme is an interesting provision of Nature for changing 

 milk into a solid form so the animal may get the full value from it. Altho 

 liquid, milk is not in condition to be taken directly into the animal sys- 

 tem, but, like solid foods, must first undergo digestion. Milk being 

 liquid, the stomach would naturally pass it quickly on to the small 

 intestine, but if this occurred it would not be sufficiently acted on by 

 the pepsin. Rennin quickly converts the milk into a solid curd which 

 is easily retained by the stomach until dissolved by the action of the 

 gastric juice. One part of rennin will coagulate 400,000 parts of milk. 

 The rennet extract used in cheesemaking is made from the membrane 

 lining the fourth stomach of young calves. 



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

 stomach, however, is so constructed that the action of ptyalin on the 

 food is not too promptly checked after the food reaches it. The first 

 portion of the stomach, into which the gullet directly leads, secretes 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 much hydrochloric acid. Here 

 the action of the ptyalin ceases, and pepsin digestion becomes active. 

 Only the preliminary steps of digestion are accomplished in the stomach, 

 and only insignificant amounts of nutrients are absorbed from it. A 

 slight digestion of fat may sometimes occur in the stomach, due to a fat- 

 digesting enzyme. 



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 the stomach contracts at the middle region, and 

 the wave of contraction proceeds slowly and regularly toward the intes- 

 tinal end, one wave following another. When digestion has progressed 

 to some extent, every time the contraction reaches the rear end of the 

 stomach, the ring of muscles which keeps the stomach shut off from the 

 small intestine relaxes and allows a small quantity of the semi-liquid 

 contents of the stomach to spurt thru into the intestine. After this the 

 ring of muscles again contracts, closing the entrance. The stomach 

 in turn slowly relaxes, and after a certain length of time, varying 

 in different animals, the. process is repeated. By this means the fluid 

 portions of the contents of the stomach are squeezed out and carried 

 into the small intestine, while the more solid portions remain behind 

 for further action by the gastric juice. In animals with a simple stomach 

 little or no churning or mixing of the food is produced by the movements 

 of the stomach. The contents are simply pushed gradually toward the 

 intestinal end of the stomach by the waves of muscular contraction. 



