There is a phenomenon that is as difficult to be well understood by this theory as any other; it is the faculty animals have of resisting external heat. Every inert body is of the same temperature as the medium which surrounds it. Every organized body on the contrary repels the caloric that tends to raise it to a higher temperature than its own. Perhaps this belongs to the laws of the propagation of caloric, of which we are ignorant.
It will be asked undoubtedly why in the ordinary state there is only disengaged a certain quantity of caloric, so as to produce an uniform temperature of a certain number of degrees of the thermometer. I answer that it is by the same cause that in the ordinary state the pulse beats nearly the same number of times in a minute, which makes common respiration consist of so many elevations and depressions of the ribs, &c. &c. It is one of those phenomena that belongs to the immutable order first established, and which it is impossible to explain. Only it appears that this immutable order depends upon the primitive type that has been impressed upon the vital forces, a type, which when nothing excites or diminishes them, produces always phenomena nearly uniform; but as a thousand causes make them vary, a thousand times the pulse, respiration, heat, &c. are capable of differing. I would observe however in regard to the last, that its variations are not so great as those of many of the other functions. Compare, for example, the ordinary quantity of secreted and exhaled fluids, with the increase that takes place under certain circumstances, the common state of the pulse with its exacerbations in many fevers, &c. you will see that between the natural and the morbid state there is often an enormous difference. The heat on the contrary, is never raised but a few degrees above the temperature of the body. When there appears by touching the parts, to be a great difference, the thermometer proves that it is in reality trifling.
I would remark, in concluding this article, that I have not sought to ascertain how the caloric is disengaged in the capillary system, what portion escapes, in what relation it is with the red and the black blood, &c.; none of these can be determined by experiment. Let us be content in our theories with establishing general principles, especially analogies between functions that are known, and those which we attempt to explain, let us attempt merely to offer some general views; but let us never hazard precise explanations. Some have endeavoured lately to determine accurately what quantity of oxygen is absorbed, what quantity goes to produce the water of respiration, what quantity of carbonic acid gas is formed, how much caloric is disengaged, &c. This precision would be advantageous if it could be attained; but no phenomenon in the living economy will admit of it, in the explanations which it occasions. Chemists and natural philosophers accustomed to study the phenomena over which the physical forces preside, have carried their spirit of calculation into the theories they have formed for those which the vital laws govern. But this should not be so. In organized bodies, the spirit of the theories should be wholly different from the spirit of the theories applied to the physical sciences. It is necessary in these last that every phenomenon should be accurately explained; that, for example, in hydraulics, all the portions of the fluids should be calculated in their motions; that, in chemistry, we should know the precise proportion and amount of each of the elements that are combined in the changes that bodies undergo.
On the contrary, every physiological explanation should give only general views, approximations; it ought to be vague, if I may use the term. Every calculation, every examination of the proportions of the fluids with each other, all precise language should be banished from it, because we yet know so little of the vital laws, they are subject to so many variations, that what is true at the moment we study a fact, ceases to be so the next, and the essence of the phenomena always escapes us; their general results only, and the comparison of these results with each other, should occupy us.
ARTICLE SECOND.
PULMONARY CAPILLARY SYSTEM.
I call by this name the assemblage of the fine and delicate ramifications, which serve for the termination of the black blood and the origin of the red, which consequently finish the pulmonary artery and give origin to the pulmonary veins. The capillaries between the bronchial arteries and veins have nothing to do with them, they have no communication, and evidently belong to the general capillary system.
I. Relation of the two Capillary Systems, Pulmonary and General.
In comparing the preceding system with this, it is difficult to understand how they can exactly correspond, how the pulmonary can transmit not only all that passes through the general, but also all the lymph that returns from the serous surfaces and the cellular cavities, all the chyle which enters by digestion, &c. &c.
It seems impossible at first view, that in the balance of the circulation, these capillaries can, constantly and regularly, keep in equilibrium with those of the rest of the body. By reflecting a little, however, upon the phenomena of this function, we see that the discordance is only apparent.