514 NUTRITION. 



the surrounding atmosphere; and there is no other nutritive act so constant and so 

 immediately necessary to existence as the appropriation of oxygen. 



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

 of discoveries. In the latter part of the last century, th4 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. 



The Consumption of Oxygen and Production of Carbonic Acid and Water in Connec- 

 tion with the Evolution of Heat. As far as it has been possible to determine by actual 

 experiment, all animals, even those lowest in the scale, appropriate oxygen and eliminate 

 carbonic acid. This is equally true of all living tissues ; and, since it has been ascertained 

 that oxygen is taken up, as oxygen, by the arterial blood, that it disappears in part or 

 entirely in the capillary circulation, that carbonic acid is taken up by the venous blood 

 to be discharged in the lungs, and that the tissues themselves have the property of appro- 

 priating oxygen and exhaling carbonic acid, those who adopt the theory of Lavoisier have 

 simply changed the seat of oxidation from the lungs to the general system. 



It has been proven beyond question that oxygen, of all the principles introduced from 

 without, is the one most immediately necessary to nutrition ; and carbonic acid is to be 

 regarded as an element of excretion, like urea, creatine, etc., differing from them only in 

 the immediate necessity for its elimination. As the comparatively slow excretion of 

 urea and other nitrogenized matters is connected with the ingestion of ordinary aliment- 

 ary substances that are slowly appropriated by the tissues, so the rapid elimination of 

 carbonic acid is connected with the equally rapid appropriation of oxygen. There is no 

 reason why we should not regard carbonic acid, like other effete substances, as an excre- 

 tion, the result of disassimilation of the tissues generally ; but, more closely than any, it 

 is connected with the rapid and constant evolution of heat. 



Experiments on the influence of the sympathetic nerves upon the temperature of par- 

 ticular parts have completed the chain of evidence in favor of the localization of the 

 heat-producing function in the tissues. It is not our purpose to discuss the relations of 

 the sympathetic system to nutrition, deferring this subject until we come to treat spe- 

 cially of the nervous system; but the facts bearing on calorification are briefly as follows: 



If the sympathetic nerve be divided in the neck of a rabbit or any other warm-blooded 

 animal, the side of the head supplied by this nerve will become from five to eight or ten 

 degrees warmer than the opposite side. This observation we have repeatedly verified. 

 The conditions under which this local exaggeration of the animal heat is manifested are, 

 dilatation of the arteries of supply of the part, so that it receives very much more blood 

 than before, and increased activity in the general process of nutrition. 



It is evident that, in normal nutrition by food, the heat of the body must be main- 

 tained by changes which take place, either directly in the blood or indirectly in the tis- 

 sues, in the alimentary matters, and that these changes involve oxidation to a very con- 

 siderable extent. Under ordinary conditions of nutrition, it is assumed that the food 

 furnishes all the material for maintaining the heat of the body and for the development 

 of force in work, such as the muscular work of respiration and circulation and general 

 muscular effort. If no food be taken for a certain time, the heat of the body must be 

 maintained and the work must be accomplished at the expense of the substance of the 

 body itself; and the individual loses weight. To furnish a positive scientific basis for 



