112 THE FEEDING OF ANIMALS 



diet, even with an animal, undoubtedly is a determina- 

 tive element in good digestion. Moreover, condiment al 

 stimulation is a poor makeshift for the effect of a healthy 

 liking for food. There are good reasons for believing that 

 psychic stimulation is an important factor in digestion 

 with farm animals. Whether the theory of chemical 

 stimuli applies to this class of animals is less certain. 



163. Digestion of food as a whole. From what has 

 preceded we learn that several liquids and certain organ- 

 isms participate in producing the complex changes that 

 food undergoes during digestion. Some of these liquids 

 have certain common functions, as, for instance, pro- 

 teins are acted upon both by the gastric and pancreatic 

 juices. Moreover, the various digesting fluids appear to 

 act cooperatively. This is made plain by following the 

 course of the food changes. 



164. Stomach digestion. After the food has remained 

 in the stomach for a certain period of time, it is gradually 

 discharged into the small intestine, the rate of discharge 

 varying with the kind of food, that is, with the prompt- 

 ness and rapidity of digestion, which differs with different 

 foods. The progress made up to this point in food trans- 

 ference, so far as we have definite knowledge, is chiefly the 

 cleavage of the proteins into various stages of hydrolysis, 

 the resulting bodies being proteoses and peptones. All 

 proteins appear to be acted on in the stomach, but to 

 different degrees and probably at different rates. Starch, 

 already somewhat dissolved by the saliva, is not further 

 acted upon by the stomach enzyms, neither are the solid 

 and liquid fats affected to any material extent. Simple 

 sugars are not acted upon by the gastric juice, but it seems 

 possible that the di-sugars may be split into simple cnes 

 by the hydrochloric acid. 



