272 VETEEINARY PHYSIOLOGY 



The critical temperature of the fed animal is therefore 

 lower than that of the fasting animal. 



3. Effect of taking Food on Metabolism. 



The consumption of food by an animal increases the basal 

 metabolism, as is seen in the following results obtained on 

 a horse : — 



Calories per hour per kg. 



Fasting 1-02 



3^ hours after feeding . . . I'lS 



The increase is influenced by the quantity and character 

 of the food. Each of the proximate principles stimulates 

 chemical changes. Proteins have a special influence in this 

 direction, and, when eaten, metabolism is so increased that 

 something like 30 per cent, of the energy they contain may 

 be liberated as heat. This is called their specific dynamic 

 action. Lusk has shown that it is due to the direct action 

 of certain of the amino-acids upon the metabolism. 



The increased metabolism following feeding was formerly 

 attributed to (1) the muscular work involved in peristalsis, 

 and (2) to the liberation of energy in the disintegration of 

 the molecules of the food in the process of digestion. Hence 

 it was said to be due to the " work of digestion," an unfor- 

 tunate term still widely used. Experiments have shown 

 that the energy expended in the mechanical work done by 

 the intestinal tract is negligible, and that the chemical 

 reactions involved in digestion are isothermic. 



It is thought that the fermentation that takes place in 

 the digestive tract of the ruminant produces a considerable 

 amount of heat. To what extent this occurs and what 

 increase follows feeding little is known. 



4. Metabolism in Prolonged Fasts. 



When the usual supply of energy in the food is cut off, 

 the animal gets the energy required by oxidising its own 

 stored material and its tissues. This is shown by the fact 

 that it loses weight and goes on excreting carbon dioxide. 



