518 NUTRITION. 



This, however, may be made to disappear by violent exercise or under an insufficient 



diet. 



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 which are not always 

 indispensable, though often very necessary articles do not form tissue, are not discharged 

 from the body, and are consumed in some of the processes of nutrition, it would seem 

 that their change must involve the production of carbonic acid, perhaps also of water, 

 and the evolution of heat. It is so difficult to ascertain the exact quantities of carbonic 

 acid, watery vapor, etc., thrown off by the lungs, skin, and other emunctories, and to esti- 

 mate the exact amount of heat produced and lost, that it is not surprising that calculations 

 of the calorific power of different articles of food should be frequently erroneous; par- 

 ticularly as we have no means of knowing the exact calorific value of the nitrogenized 

 principles. 



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 

 single 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), and that simply in solution in the blood. There can be 

 no doubt that, of all the nutritive acts, respiration is, far more than any other, intimately 

 connected with calorification. As far as the general process is concerned, the production 

 of heat is usually in direct ratio to the consumption of oxygen and the exhalation of car- 

 bonic 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 oxygen 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 the surrounding temperature ; and there is no 

 other nutritive act so constant and so immediately necessary to existence as the appro- 

 priation of oxygen. It is not surprising, then, that, early in the history of the physiology 

 of nutrition, before we knew even the exact condition and proportion of the gases in 

 the blood, it should have been thought that animal heat was the result of slow combustion 

 of the hydro-carbons. 



The physiological history of respiration and of animal heat dates from the same series 

 of discoveries. In the latter part of the last century, the great chemist, Lavoisier, discov- 

 ered the intimate nature of the respiratory process and applied the theory of the con- 

 sumption of oxygen and the evolution of carbonic acid to calorification. Like nearly all 

 of the great advances in physiological science, the distinctly-enunciated idea was fore- 

 shadowed by earlier writers. It will not be necessary to treat, from a purely historical 

 point of view, of the discoveries made by Lavoisier. He undoubtedly went as far in his 

 explanations of the phenomena of animal heat as was possible in the condition of the 

 science at the time his investigations were made ; and, although he inevitably fell into 

 some errors in his calculations and deductions, he must forever be regarded as the author 

 of the first reasonable theory of the generation of heat by animals. 



