184 THE FEEDING OF ANIMALS 



external motion (work) and partly into heat, and this 

 certainly is consistent with facts as observed. Violent 

 exercise by the animal greatly increases the production 

 of heat. We know this is so because under these con- 

 ditions an increased amount of blood is thrown to the 

 surface of the body, thereby greatly increasing the loss 

 of heat by radiation; perspiration sets in and with it the 

 consequent evaporation of much more moisture, thus 

 disposing of much heat. The dog, and sometimes other 

 animals, pants and thereby causes a large loss of heat 

 from the expanded surface of the moist tongue. AU this 

 occurs without reducing the body temperature below 

 the normal. In fact, nature adopts these various devices, 

 such as increased circulation of the blood and perspira- 

 tion, in order to regulate the body temperature and pre- 

 vent its rising above the proper point. The explanation 

 of this greater heat during labor is that the mechanical 

 energy manifested by the muscles is converted to heat, 

 which under circumstances of severe exercise is more 

 than enough to keep the body at its usual temperature 

 and maintain the usual radiation. When it is severely 

 cold, on the other hand, vigorous exercise is sometimes 

 necessary in order to keep sufficiently warm. 



267. Animal heat a secondary or waste product. — 

 The view now obtains that under certain conditions 

 body heat is wholly a secondary product, that combustion 

 first supports muscular activity with heat as a by-pTod- 

 uct; in fact, that at usual temperatures no food is 

 burned primarily to keep the animal warm. Under cer- 

 tain conditions there may be combustion of food for the 

 specific purpose of warming the body. In any case, 

 animg,l heat is sustained either directly or indirectly by 

 the oxidation of the nutrients. 



