Not only is the temperature of the muscular system itself raised, but the rapidity of the circulation is accelerated, a larger quantity of warm blood is brought to the skin in a giventime, and the sensitive integument thus feels the increased temperature. No doubt it is owing to this fact that active muscular exercise is itself a protection against external cold. An unusual degree of heat in the atmosphere also tends indirectly to raise the temperature of the body; for if the internal production of heat be the same, and its external loss by contact with the atmosphere be diminished, of course the actual temperature of the body would rise in consequence. A provision is made, however, against allowing this increase of temperature, whether from muscular exertion or external heat, to reach too high a point. This provision is the cutaneous perspiration. Anything which raises the bodily heat above the natural standard excites the circulation through the skin, and increases the quantity of perpsiration poured out upon its surface. This fluid, by its evaporation, uses up or renders latent a portion of the heat, and thus reduces the skin and the blood circulating through it to its natural temperature.

The body therefore can be exposed to' a very high external temperature without itself rising above its natural standard, provided the perspiration be free and its evaporation unimpeded. If the perspiration be checked, however, or if its evaporation be prevented by exposure to hot water, or hot air loaded with moisture, the temperature of the body rises, and death soon takes place. The experiments of Magendie and others have shown that in the higher animals life is destroyed when the blood generally has become heated 10° or 13° above the natural standard. Animals therefore have a natural internal temperature, which is essential to the performance of the vital functions, and which cannot be either raised or lowered to any considerable extent without producing death. - With regard to the precise mode in which animal heat is generated, and its exact chemical conditions, opinions are not entirely agreed. Many physiologists have entertained and still accept the belief that it is due to an oxidation or combustion of the elements of the blood and tissues by the oxygen absorbed in respiration.

The grounds for this doctrine are as follows: 1. The most common and ready method by which heat is generated artificially is the combustion of substances, like wood and coal, which are rich in carbon. The rapid oxidation of these substances, which requires a free access of air, causes a great development of heat, and at the same time uses up the oxygen of the atmosphere, and produces as a result carbonic acid. The consumption of fuel, the degree of heat produced, and the quantities, of oxygen absorbed and carbonic acid liberated, are all in direct ratio to each other. The process may go on rapidly or slowly; but in either case the relations of quantity remain the same. If the oxidation be rapid, as in a furnace or open fireplace with a strong draught, the fuel is soon consumed and a large quantity of heat is produced in a given time. If the process be retarded, as in a close stove with a limited or gradual admission of air, the consumption of fuel is slow, and the heat, less intense at any particular moment, is continued for a proportionally longer time.

But in both instances, for the entire amount of heat which has been generated, there are the same quantities of fuel consumed, of oxygen absorbed, and of carbonic acid produced. 2. In the animal body the absorption of oxygen and the exhalation of carbonic acid are the most striking and constant of all the phenomena of nutrition. At the same time heat is evolved, as in the case of artificial combustion; and it is very natural to connect the two sets of phenomena with each other. Furthermore, as in artificial combustion, the elevation of temperature in different animals corresponds very closely with the activity of respiration and the quantity of the two gases inspired and exhaled. These considerations have led to the adoption of the theory, at once intelligible and comprehensive, which attributes the production of animal heat to the direct oxidation or combustion of the carbonaceous ingredients of the food and tissues. - On the other hand, there are certain facts which are less favorable to the above theory. 1. In the first place, though the combustion of carbonaceous matter happens to be the most familiar and useful of the artificial means for producing heat, it is by no means the only one which will have that effect.

A great variety of both physical and chemical changes, other than oxidation, are attended with an elevation of temperature, often of a very active kind; as in the ordinary slaking of lime, where a boiling temperature may he reached in a few minutes by the simple combination of water with the alkali, which already contains the oxygen it is capable of absorbing. A great variety of chemical and physical changes are constantly going on in the process of nutrition, varying in their character in the different organs; and of their details we are in many cases still ignorant. As we have seen that animal heat is produced as a local phenomenon in the different organs, it may be the result of these combined changes, which vary in character in different parts of the body. 2. The first absorption of oxygen by the blood, which takes place in the lungs, is not accompanied by any very marked elevation of temperature. This elevation, if it exist at all, is not sufficient to compensate for the cooling effect of the air and exhalation in the pulmonary cavities; for we have seen that in the living animal the blood has been found by experiment to lose slightly instead of gaining in temperature while passing through the lungs.

The oxygen is here taken up by the red blood globules, and thence distributed to the tissues; but it is doubtful whether its subsequent transfer to the ingredients of the tissues has any more the character of an active combustion than its first absorption by the blood. Some physiologists regard oxygen as a kind of food which must be supplied to the body with great regularity and constancy, and which is destined to become a constituent part of the tissues very much in the same manner as other nutritive elements. 3. The production of carbonic acid in the interior of the body is directly due, not to a combination, but to a decomposition of the ingredients of the tissues. Carbonic acid may be generated at anytime in either of two ways: by the immediate combination of oxygen with carbon, as in the combustion of charcoal; or by the decomposition of another body still more compound in its nature, as in the decomposition of carbonate of lime by an acid, or the decomposition of sugar in fermentation. In both these latter cases carbonic acid is evolved without any direct oxidation taking place; and the process will go on accordingly without the access of oxygen or atmospheric air.

In the animal body it is by such a process of decomposition that carbonic acid is produced; and the proof of this is, that if the fresh muscles of a frog, or the living animal itself, be enclosed in an atmosphere of hydrogen or nitrogen, or even in a vacuum, they will still for a considerable period continue to exhale carbonic acid. This has been fully shown by the experiments of Marehand. 4. While it is true that the development of animal heat is in proportion to the consumption of oxygen and the exhalation of carbonic acid, this is also true of most if not all the other substances consumed and eliminated by the living body. An abundant production of warmth coincides with a general vigor and activity of all the animal functions, with muscular exertion, capacity of endurance, and a liberal consumption of both the nitrogenous and non-nitrogenous elements of food. We cannot safely attribute the heat-producing power exclusively to one or the other class of alimentary substances; for while fat and albuminous matters are both consumed in large quantities in cold climates, on the other hand starchy materials form a considerable proportion of the food in warm weather and in tropical climates.

In point of fact, oxygen and carbonic acid are two substances which enter and are discharged from the system by the same organ, the lungs; but there is not necessarily any direct relation between them, except that oxygen is one of the nutritious substances essential to the body, and carbonic acid is excrementitious.