EVOLUTION OF HEAT. 539 



The blood is dark and contains little oxygen (1 or 2 per cent,). Both Dr. 

 Davy 1 and Pouchet found that completely congelated animals were incapa- 

 ble of being restored to life. Accurate experiments are yet wanting to de- 

 termine the influence of humidity on the effects of cold air. From experi- 

 ments on young Birds incapable of maintaining their own temperature, of 

 which some were placed in cold dry air, and others in cold air charged with 

 moisture, it was found by Dr. Edwards that the loss of heat was in both 

 instances the same ; the effect of the evaporation from the surface in the 

 former case, being counterbalanced in the latter by the depressing influ- 

 ence of the cold moisture. This influence, the existence of which is a mat- 

 ter of ordinary experience, is probably exerted directly upon the Nervous 

 system. 



430. Having thus considered the general facts which indicate the faculty 

 possessed by the living system, in Man and the higher Animals, of keeping 

 up its temperature to an elevated standard, and of preventing it from being 

 raised much beyond it by any degree of external heat, we have next to in- 

 quire to what this faculty is due. 2 It may be stated as a general fact, that 

 every change in the condition of the organic components of the body, in 

 which their elements enter into uew r combinations with Oxygen, must be a 

 source of the development of Heat. And as we have seen that a consider- 

 able part of the carbonic acid and water which are exhaled in Respiration, 

 is formed within the body by the metamorphosis of its own tissues, and that 

 this metamorphosis is promoted by the active exercise of the nervo-muscular 

 apparatus, it follows that in animals whose habits of life are peculiarly 

 active, whilst the temperature of the surrounding medium is sufficiently 

 high to prevent its exerting any considerable cooling influence over them, 

 the combustive process thus maintained may be adequate for the mainte- 

 nance of the temperature of the body at its normal standard. This seems 

 to be the case with the great Carnivorous quadrupeds of warm climates, 

 and with certain races of Men who lead a life of incessant activity like 

 theirs. But whenever the cooling influence of the atmosphere is greater, or 

 the retrograde metamorphosis of tissue takes place with less activity, some 

 further supply of heat-producing material is required ; and this is derived 

 either directly from the food, or from a store previously laid up in the body. 

 Although the albuminous and gelatinous components of the food may be 

 made, by decomposition within the body, to yield saccharine and oleaginous 

 compounds which serve as an immediate pabulum to the combustive process 

 ( 78, 344), yet this metamorphosis involves a great waste of valuable nu- 

 tritive material ; and the needed supply is much more advantageously de- 

 rived at once from those farinaceous or oleaginous substances, which are 

 furnished in abundance by the Vegetable kingdom, the latter also by Ani- 

 mals. No reasonable doubt can any longer be entertained, that the pro- 

 duction of Heat by the combustive process is the purpose to which a large 

 proportion of these substances is destined to be subservient in the bodies of 

 Herbivorous animals and of Man ; and the results of experience in regard 

 to their relative heat-producing powers, are in precise accordance with the 

 indications afforded by their chemical composition. 



431. Our knowledge of the dependence of all the vital processes in warm- 



1 Proceed of the Koy. Soc., 1866, p. 250. 



2 It was affirmed by Dr. Granville (Phil. Trans., 1825) that the temperature of 

 the uterus during parturition sometimes rises as high as 120. In some observations 

 made at the Philadelphia Hospital, however, at the desire of Prof. Dunglison, the 

 temperature of the uterus was not I'ound to be much above that of the vagina; the 

 former being, in three cases, 100, 102, and 106, whilst the latter was 100, 100, 

 and 105. (Prof. Dunglison's Human Physiology, 8th edit., vol. i, p. 602 ) 



