60 SCIENTIFIC FEEDING OF ANIMALS 



may be filled with the products of protein digestion 

 there are none of these substances to be detected 

 in its walls even after the absorption of all the 

 contents of the intestines. These changes may be 

 said to be effected by the help of enzymes, and the 

 proteins which result closely resemble those of the 

 animal body. The splitting up of the different 

 food proteins by the digestive fluids yields in this 

 way the material from which the body protein is 

 built up. So it is clear that the food proteins, which 

 differ considerably from the tissue proteins, are 

 nevertheless essential materials for the building up 

 of them. In the above metabolic changes waste 

 products are formed, as they are in the process of 

 digestion. 



Numerous feeding experiments have shown that 

 no animal can live unless it gets a certain quantity 

 of protein. All other nutrients may be there in 

 abundance, but if protein fails, partially or entirely, 

 the animal draws daily upon its own body for 

 a supply, and finally dies from protein starva- 

 tion. 



The way in which this takes place when different 

 amounts of protein are given is seen from the 

 following experiment. 



A fully-grown dog which weighed 30 kg. (66 lbs.) 

 used up the following quantities of body protein 

 and body fat when fasting and when being fed with 

 lean meat, which is almost pure protein. 



