THE PRODUCTION AND DISSIPATION OF HEAT 1093 



amount of heat which, however, is never considerable. Inasmuch as 

 the law of the conservation of energy is directly applicable to the ani- 

 mal body, these processes must serve to convert latent or potential 

 energy into its kinetic form. Consequently, the ultimate source of 

 heat is the potential energy of the food, and it matters little whether 

 this material be. slowly oxidized in the body or be burned up in a 

 calorimeter. In both cases, complex substances are reduced into 

 relatively simple bodies under an evolution of energy which manifests 

 itself as mechanical energy, heat, and electricity. This combustion, 

 however, remains incomplete at times, as is shown, for example, by 

 the proteins which always leave a residue of urea, uric acid and other 

 substances. Consequently, any calculation of the total heat-energy 

 of a given foodstuff must take into account the energy of this possible 

 residue. Carbohydrates and fats, on the other hand, are oxidized 

 completely. 



Body-temperature. Since all the tissues of our body take part in 

 these processes of oxidation, every cell in our body may be said to be 

 a producer of heat. Admittedly, however, tissues differ very markedly 

 in their activities, and hence, also in the quantity of heat evolved by 

 them. The most important heat-generating organ is the skeletal 

 musculature, because it is hardly ever at rest, and because more than 

 one-half of the total weight of the soft parts of our body is made up of 

 this type of tissue. Next in order are the glands. In either case, 

 an active organ is always warmer than a resting organ or the body- 

 fluids, and hence the heat must be transmitted from the former to the 

 latter. In the nature of things, the chief and final heat absorbing 

 agent is the blood which by virtue of its velocity quickly removes the 

 superfluous heat from the seats of the oxidations, thereby tending to 

 keep the temperature approximately uniform throughout. Later on, 

 when the blood enters the more exposed portions of our body, it loses 

 a part of its heat either by radiation or in the form of bound heat. 



It is evident, therefore, that the blood and lymph form a medium 

 into which the different tissues pour their heat and which by virtue of 

 its motion tends to equalize the temperature of the different parts of 

 the body, and also to bring the latter into proper heat-relation with 

 the surrounding air. In accordance with their resistance to outside 

 influences, animals may be divided into two classes, namely, into those 

 which are and those which are not protected against a loss of heat. 

 This functional difference brings it about that the former are capable 

 of retaining a relatively constant temperature in spite of the fact that 

 the outside temperature may vary considerably, while the latter are 

 not and must, therefore, be subject to the fluctuations of the tempera- 

 ture of the medium in which they live. The first are designated as 

 constant-temperatured, homoiothermal or warm-blooded animals, and 

 the se'oond as inconstant-temperatured, poikilothermal, or cold-blooded 

 animals. But sirioe the poikilothermal animals may be made to attain 

 a body- temperature equalling and even surpassing that of the homoio- 



