ANIMAL HEAT. 513 



and it is almost certain that non-nitrogenized matter is not produced in the organism in 

 quantity sufficient to account, by its destruction in the lungs, for the carbonic acid exhaled. 

 It seems beyond question that there must be heat evolved in the body by oxidation of 

 nitrogenized matter. When the daily amount of food is largely increased for the purpose 

 of generating the immense amount of heat required in excessively cold climates, the nitro- 

 genized matters are taken in greater quantity, as well as the fats, although their increase 

 is not in the same proportion. From these facts, and from other considerations that have 

 already been fully discussed, it is evident that the physiological metamorphoses of ni- 

 trogenized matter bear a certain share in the production of animal heat. 



What is the relation of the consumption of non-nitrogenized matter to the production 

 of animal heat ? 



It has been pretty clearly shown that both sugar and fat are actually produced in the 

 organism, even when the diet is strictly nitrogenized in its character ; but we shall con- 

 sider only the relations of the non-nitrogenized elements introduced into the body, assum- 

 ing that the principles of this class appearing de now in the organism are the result of 

 a transformation of nitrogenized substances. 



As far as the destination of the amylaceous, saccharine, and fatty elements of food is 

 concerned, we only know that they are incapable, of themselves, of repairing muscular 

 tissue, and that they cannot sustain life. They are never discharged from the body in 

 health in the form under which they enter ; but they are in part or completely destroyed 

 in nutrition. They are completely destroyed in persons who, from habitual muscular ex- 

 ercise, have very little adipose tissue. When their quantity in the food is large, they are 

 not of necessity entirely consumed, but they may be deposited in the form of adipose 

 tissue. 



There can be no doubt that the non-nitrogenized class of alimentary principles is 

 craved by the system in long-continued exposure to extreme cold. This is particularly 

 marked with regard to the fats. In all cold climates, fat is a most important element of 

 food ; and, in excessively cold regions, while the nitrogenized elements are largely in- 

 creased, there is a very much larger proportional increase in the quantity of fat. These 

 facts are very significant. If the non-nitrogenized elements of food do not form tissue, 

 are riot discharged from the body, and are consumed in some of the processes of nutri- 

 tion, it would seem that their change must involve the production of carbonic acid, per- 

 haps also of water, and the evolution of heat. 



Although we may assume that the non-nitrogenized elements of food are particularly 

 important in the production of animal heat, and that they are not concerned in the repair 

 of tissue, it must be remembered that the animal temperature may be kept at the proper 

 standard upon an exclusively nitrogenized diet; and we cannot, indeed, connect calorifi- 

 cation exclusively with the consumption of any single class of principles or with any sin- 

 gle one of the acts of nutrition. 



Relations of Calorification to Respiration. Respiration is one of the nutritive pro- 

 cesses that can be closely studied by itself, as it involves the appropriation by the system 

 of a single principle (oxygen), which is carried to the tissues by the blood. There can be 

 no doubt that, of all the nutritive acts, respiration in the substance of the tissues is, far 

 more than any other, intimately connected with calorification. As far as the general pro- 

 cess is concerned, the production of heat is usually in direct ratio to the consumption of 

 oxygen and the exhalation of carbonic acid. In the animal scale, wherever we have the 

 largest amount of heat produced, we observe the greatest respiratory activity. In man, 

 whatever increases the generation of heat increases as well the consumption of oxygi-n 

 and the elimination of carbonic acid. The production of heat in warm-blooded animals 

 is constant, and it cannot be interrupted, even for a few minutes. The same is true of 

 respiration. The tissues may waste for want of nourishment, but the heat of the body 

 must be kept near a certain standard, which is almost always much higher than that of 

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