508 PHYSIOLOGY. 



tions have both the greatest velocity and the greatest amplitude. 

 Hence, heat is not a substance, but a condition of matter. It is a 

 condition which can be transferred from one body to another. When 

 a heated body is p.aced in contact with a cooler one, the former gives 

 more molecular motion than it receives; but the loss of the former 

 is the equivalent of gain of the latter. 



Animal Heat. Within the organs of the human body, as well as 

 those of all animals, processes of oxidation are continually going on. 

 0::ygen passes through the lungs into the blood to be thus carried to 

 all parts of the body. In like manner the oxidizable bodies, which 

 are principally foods, pass by the processes of digestion into the 

 blood finally to reach every part of the body. The gases, liquids, 

 and solids which enter the body are loaded with energy. These 

 various bodies are intimately concerned in the different chemical 

 processes which sum up metabolism: that is, those phenomena 

 whereby living organisms are capable of incorporating into their tis- 

 sues, substances obtained from their food. Metabolism is also con- 

 cerned in the formation of a store of potential energy which may 

 readily be transformed into kinetic energy, as manifested in muscular 

 work and heat. Within the body the assimilable substances undergo 

 many chemical changes, and finally leave it in forms quite different 

 from those on entering it. The oxygen inspired combines mainly with 

 carbon and hydrogen to form carbon anhydride and water, while the 

 more complicated compounds are reduced to simple bodies, to be 

 excreted as such. In the process of disintegrating these compourds 

 in fact, in catabolism in general one of the most important re- 

 sults is the production of heat. The energy enters the body as poten- 

 tial energy stored up in the food. By chemical processes it becomes 

 evolved into kinetic energy and heat. Animal heat is the accompani- 

 ment of the formation of carbonic acid, urea, and other excreted 

 products. According to our theory of heat, the animal heat due to 

 metabolic processes must represent to us vibrations of the corporeal 

 atoms. 



Other Sources. Roughly speaking, the muscles constitute about 

 one-half of the whole mass of the body, the bones the other half. As 

 but little oxidation occurs in the bones, the muscles must be the 

 chief seat of heat-production. Muscular exercise greatly increases 

 the metabolism and the C0 2 excreted, but there is an accompanying 

 increase in heat-production. In health the muscles yield four-fifths 

 of the body heat. 



The secreting glands are known to be centers of thermogenesis 



