172 FOODS [chap. 



the animal. On the other hand, there is plenty of evi- 

 dence that the fat of the food never passes into the body 

 or into the milk without some change. Once digested, 

 all fat that is not stored is used by the animal as fuel in 

 order to maintain its heat and provide it with the 

 energy by which it is enabled to work. In this process 

 the fat is completely burnt up, and its elements leave 

 the body again in the form of carbon dioxide and water, 

 having undergone just the same change as would have 

 taken place had the fat been set on fire. The fat is 

 truly burnt, and has been proved to give out the same 

 amount of heat inside the animal as if it had been 

 burnt in a lamp. 



The carbohydrates contained in the food also 

 become soluble as a result of enzyme action ; the sugars 

 are, of course, soluble to begin with, but starch is first 

 attacked by a diastase which occurs in the secretion of 

 the saliva. In consequence, the digestion of starch is 

 active in the first stomach and during the " chewing of 

 the cud " in ruminant animals. This salivary digestion 

 is, however, suspended in the stomach proper, because of 

 the acid reaction there set up ; but the starch is attacked 

 again in the small intestine, after the influx of the 

 alkaline bile and the pancreatic juice, which gives rise 

 to a second diastatic enzyme. The digestion of cellulose 

 and the more soluble portions of the fibre is less easy to 

 follow. All seeds themselves contain an enzyme capable 

 of attacking cellulose, and this would become active in 

 the alimentary canal, especially in the long digestive 

 tract of ruminants, in which the food remains for four or 

 five days. There is little evidence that animals them- 

 selves secrete any cytase or cellulose-dissolving enzyme. 

 The intestinal tract, however, contains bacteria, which 

 multiply greatly in the food, and are able to break 

 down the cellulose and fibre into various simpler bodies, 



