Page:Popular Science Monthly Volume 2.djvu/422

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tive force by any voluntary operation, which would be one of the prerogatives of life; they draw it from the calorific energy stored up in the organs traversed by the blood. Besides, there is a fixed relation between the quantity of heat that disappears and the mechanical labor that appears. Yet, it is to be remarked that, if all motion by living beings is a transformation of animal heat, that heat is not wholly transformed into motion. It is partly wasted by transpiration through the skin, by touch, and especially by radiation; it is used in keeping up to a constant point the temperature of the animal, subjected to many causes of refrigeration.

The mechanical labor performed by an animal is very complex. Independently of visible muscular motions, there are all the changes of place in the interior organs, the continual passage of the blood, the contractions and dilatations of a great number of parts. Now, these actions are only possible in so far as the phenomena of breathing are taking place in the active region. Prevent arterial blood from coming to the muscle, that is to say, prevent combustion taking place, and consequent heat evolving in it, and, although the structure of the organ suffers no harm, it loses its contractile power. Mere compression of the supplying artery of the muscle, so as to check the flow of blood in it, causes the organ to grow cool, and lose its power. The labors of Hirn and Béclard have clearly established the relations between heat and muscular motion. Later experiments by Onimus have fixed, with equal precision, the efficiency of heat through the movements of circulation.

We have said that the heat-producing power of aliments will be the more considerable in proportion as they contain a greater quantity of elements that need a large supply of oxygen for their combustion. Therefore, meat and fats repair the losses of the system much more speedily than vegetable substances. The latter are suitable for the inhabitants of warm countries who do not require to produce heat, which the atmosphere supplies them with abundantly. The inhabitants of cold regions, on the contrary, whose accessions of heat ought to be as continual as energetic, are urged by instinct to use meats and fats, which throw out great heat in their combustion. For instance, it is a physiological necessity that the Lapps should feed on the oil of cetacea, as it is a necessity for men of the tropics to consume only very light food. The activity of respiratory combustion and the kind of alimentation thus vary with climate, so that there is always a certain proportion maintained between the thermic state of the surrounding medium and that of the animal furnace. In like manner, in the same climate, persons who perform great mechanical labor must eat more than those who put forth but little movement. This fact, long ago observed, has received of late the clearest and surest demonstration. Yet, perhaps, it is not kept sufficiently in view in the management of public alimentation. Many examples prove the benefit that industry would de-