We now proceed to the third division of the subject, the effects of respiration upon the living system. As the generation of animal heat has been supposed to be one of the most important of these, we shall inquire into the na ture of this function.
One of the most obvious circumstances in which differ ent classes of animals differ from each other, is their tem perature. All those which possess the greatest variety- of organized parts, and whose functions arc the most perfect, possess the power of maintaining their temperature at a uniform standard, and one considerably above that of the atmosphere. The temperature of birds is the highest, that of man and the mammalia is about 98°, and these ex perience little alteration, except near the surface, while fish and reptiles possess what is called cold blood, being of a variable temperature, only a degree or two above that of the media in which they are immersed. The subjects of inquiry respecting animal heat, arc, 1. What is the cause or source of it ? 2. By what means is its uniformity preserved ? To which we may add, in the 3d place, How is the body cooled, when placed in a temperature higher than that which is natural to it ? Of all the functions of the living body, none is, perhaps, more calculated to excite our admiration than its power of prouucing heat. The body is surrutinued by an atmos phere generally 40 or 50 degrees colder than itself, and heat must necessarily al all times be rapidly abstrae,ed from it ; yet a supply is found sufficient to repair the loss. This appeared to the ancients so much beyond the reach of all physical action, that they attributed animal heat to the direct agency of Omnipotence itself; or it was spoken of as something mysterious or inexplicable, beyond the reach of human intellect to explain or comprehend. It re mained for the present age to attempt the solution of this curious problem.
With respect to the cause of animal heat, the ancients were so far from adopting any rational views respecting it, that, as we have just remarked, they seem to have re garded it as scarcely forming a legitimate subject for phy siological research. Galen says that it is a primary innate quality of the body, contemporary with life, and that it has its origin in the heart ; indeed, so far was he from con ceiving that it originated in the lungs, that he considered the cooling of the blood to be the principal use of the function of respiration. After the revival of letters, when physiologists were divided between the chemical and the mechanical sects, the doctrine of animal heat was viewed in various lights, according to the theory of the writer.
By some it was supposed to depend upon the fermenta tion of the blood; while others imagined that it was pro duced by the friction of its particles, as they passed through the minute vessels ; and the effect of respiration, in both cases, was not to impart heat, but to remove it from the system.
Although particular expressions may be occasionally met with which hint at a more correct opinion, the first real information that we obtain upon this subject, is de rived from the researches of Black on latent heat. After having shown that bodies indicating the same temperature to the thermometer, actually contain different quantities of caloric, he proceeded to apply this principle to explain the phenomena of combustion, and other chemical pro cesses, in which heat is extricated. In the burning of charcoal, this substance is united to oxygen, and forms carbonic acid ; while, at the same time, heat is generated. Here we conclude, that the oxygen and carbon, in their separate state, have a greater capacity for heat than the same bodies when united in the form of carbonic acid ; so that a part of their capacity is destroyed upon their union. After he had found that the same kind of chemical change is produced by respiration as by combustion, and that heat is also generated, he concluded that it was due to the same cause, to the formation of carbonic acid ; and hence, that respiration was the cause or source of animal heat, being literally a species of combustion.
Black's conclusion was received as the fair deduction from the facts, and was regarded as at least a considerable approximation to the true nature of the hitherto inexplica ble function; yet it was found that there were still formi dable difficulties to be removed, before it could be adopted as a correct theory. It was objected, and apparently with justice, that if the union of the carbon and oxygen took place in the lungs, their temperature ought to be much greater than that of any part of the body, whereas all the internal organs were supposed to be nearly at the same temperature ; and this objection appeared so forcible, as almost to induce Black himself to relinquish the theory, Nyhen the investigation was taken up by Crawford The result was, the formation of an hypothesis, professing to be the direct result of experiment, which seemed to re move the difficulties that had occurred to Black, and afforded one of the most beautiful speculations which had hitherto been presented to the world on any subject con nected with the animal economy.