METABOLISM AND FOOD 81 



heat value of these two substances was stored up 

 as body fat, whereas from the cane sugar only 45% 

 was stored. The smaller return obtained from 

 the sugar is doubtless to be explained by the fact 

 that this material is very easily soluble and there- 

 fore more largely attacked by the bacteria in the 

 intestines than are the starch or crude fibre. 



It is well known that solutions of sugar readily 

 become sour when mixed with fermenting sub- 

 stances, and it has been observed that shortly after 

 ruminants have eaten sugar there is very little to be 

 detected in the partially digested food, but, on the 

 other hand, considerable quantities of lactic acid. 



With pigs, in whose alimentary canal the bac- 

 terial activity is much less, and also with horses, 

 most probably the sugar gives a much more favour- 

 able return than with ruminants. Lactic acid is 

 not able, as has been shown from investigations on 

 this point, to undergo conversion into body fat, so 

 it can only serve as a source of heat to the animal. 



If the results of all the investigations which have 

 been made on the action of digestible components 

 of the food were put together they could be ex- 

 pressed in the following sentences — 



(a) Flesh is formed in the body principally from 

 protein. Amongst the nitrogenous substances of 

 non-protein nature there are some which can be 

 changed in the intestines with the help of bacteria 

 into protein, and so take part in the formation of 



G 



