312 HAND-BOOK OF PHYSIOLOGY. 



It is not necessary to assume that the combustion processes, which 

 ultimately issue in the production of carbonic acid and water, are as sim- 

 ple as the bare statement of the fact might seem to indicate. But com- 

 plicated as the various stages of combustion may be, the ultimate result 

 is as simple as in ordinary combustion outside the body, and the products 

 are the same. The same amount of heat will be evolved in the union 

 of any given quantities of carbon and oxygen, and of hydrogen and oxy- 

 gen, whether the combination be rapid and direct, as in ordinary combus- 

 tion, or slow and almost imperceptible, as in the changes which occur in 

 the living body. And since the heat thus arising will be distributed 

 wherever the blood is carried, every part of the body will be heated equally, 

 or nearly so. 



This theory, that the maintenance of the temperature of the living 

 body depends on continual chemical change, chiefly by oxidation, of 

 combustible materials existing in the tissues, has long been established by 

 the demonstration that the quantity of carbon and hydrogen which, in 

 a given time, unites in the body with oxygen, is sufficient to account for 

 the amount of heat generated in the animal within the same time: an 

 amount capable of maintaining the temperature of the body at from 98 

 100 F. (36-8 37'8 C.), notwithstanding a large loss by radiation and 

 evaporation. 



It should be remembered that heat may be introduced into the body 

 by means of warm drinks and foods, and, again, that it is possible for the 

 preliminary digestive changes to be accompanied by the evolution of heat. 



Chief Heat-producing Tissues. The clfemical changes which 

 produce the body-heat appear to be especially active in certain tissues: 

 (1), In the Muscles, which form so large a part of the organism. The 

 fact that the manifestation of muscular energy is always attended by the 

 evolution of heat and the production of carbonic acid has been demon- 

 strated by actual experiment; and when not actually in a condition of 

 active contraction, a metabolism, not so active but still actual, goes on, 

 which is accompanied by the manifestation of heat. The total amount 

 set free by the muscles, therefore, must be very great; and it has been 

 calculated that even neglecting the heat produced by the quiet metabolism 

 of muscular tissue, the amount of heat generated by muscular activity 

 supplies the principal part of the total heat produced within the body. 

 (2), In the Secreting glands, and principally in the liver as being the 

 largest and most active. It has been found by experiment that the blood 

 leaving the glands is considerably warmer than that entering them. The 

 metabolism in the glands is very active, and, as we have seen, the more 

 active the metabolism the greater the heat produced. (3), In the Brain; 

 the venous blood having a higher temperature than the arterial. It 

 must be remembered, however, that although the organs above mentioned 

 are the chief heat-producing parts of the body, all living tissues contribute 



