THE DIGESTIVE SYSTEM 



109 



However, examination shows that while it is abundant in butter- 

 fat it is absent in lard, tallow, and other body fats, and also in 

 cottonseed oil, a vegetable fat. Therefore, these fats cannot be 

 substituted for butter-fat. 



While these epoch making discoveries have not undermined the 

 old idea that fats are a concentrated source of energy, which the 

 animal stores in a time of liberal feeding to be drawn upon in 

 a time of need as a source of heat or work energy, they do prove 

 that all fats are not of equal value. Moreover, only fats from 

 certain definite sources influence growth and reproduction. 



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Fig. 33. — Showing the number of pounds of protein which a pig can retain 

 for growth from each 100 pounds of protein consumed, when the proteins are 

 derived from the different sources indicated. (Wisconsin Bulletin 291.) 



Important as are these two dietary substances, it should be 

 clearly understood that they are no more so than are the proteins, 

 carbohydrates, water, and mineral constituents of the ration. 

 For example, if any one of the eighteen or more amino-acids, 

 which represent the cleavage products of protein digestion, is lack- 

 ing, starvation will eventually follow. Figs. 32 and 33 illustrate 

 variations in the nutritive value of proteins from different sources. 



A series of experiments in which cows and pigs were used 



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