But there is one source of inquiry into the laws of animal heat which has been little dipped into, although it is beyond all comparison the most abundant. I allude to that presented to us by nature in the all but infinite variety of modifications of organization and pheno mena exhibited in the vast chain of animated things, not only in the diversities of species, but also in the varieties of age and constitution and the changes induced by the states of healtt and disease. In making this an object o peculiar study, we become acquainted wit! the greatest possible number of phenomena connected with animal heat ; and in determin ing the physiological conditions of its produc tion, we shall lay up a store of theoretical knowledge peculiarly applicable to practice, the end and object of all physiological inves tigation.
The means of comparing these modifications, however, and of judging of their importance are not always easy. We shall do as much as the actual state of our knowledge permits if we inquire first, by what means we can ap preciate the modifications relative to the arterial blood.
1. As regards the quantity of the arterial blood, we shall view this point of the inquiry less with reference to the whole amount of blood circulating in the body, than to the quantity which is formed at a time, as it were, in the lungs; because it is evident that if the arterial blood influences the phenomenon of heat, the more that is formed at any given time the greater ought to be the direct or in direct influence upon the production of heat. a. As it is not always possible to have a direct and precise measure of the relative quantity of blood in the organs, we must be content with an approximative mode of estimating this, which consists in ascertaining in what degree the lungs are loaded with blood. b. An aid to the judgment may also be derived from the relative size of the lungs, the tissue being presumed to be nearly alike throughout their entire mass. e. With an equal volume of lungs, the greater or less compactness of the tissue must be taken into the account. The closer the tissue is, the more are the surfaces in contact with the air multiplied. d. The extent and rapidity of the respiratory motions form another element in the calculation ; for to increase the amount of relation with the air is analogous to the for mation of a larger quantity of arterial blood within a given time.
All the foregoing data refer to the absolute or relative quantity of arterial blood. But there are other particulars connected with its constitution which it is necessary to mention.
The blood, for instance, is composed of a fluid and solid part, the latter existing under the form of globules. It is obvious that the fluid is not the characteristic part of the blood, in asmuch as this is met with elsewhere, whilst the globules of the blood are only known as constituents of this fluid. The arterial blood consequently ought to hove qualities by so much the more distinctive and energetic as it con tains a larger proportion of globales. Now this is a character that may be appreciated with exactness, and measures of it have been given. But the globules of the blood are not in variably of the same nature, a fact which may be judged of by outward and very obvious and appreciable characters, namely, size and form. The smallness and more or less perfectly sphe rical or rounded form of the blood-globules distinguishing animals with warm blood, co incide in the Vertebrata with a higher capacity to produce heat. For we do not institute this comparison here save in reference to animals included in this division, inasmuch as the cha racters of the blood have only been studied under these relations among them. We shall, therefore, hold the energy of the calorific power to be connected with the smallness and rounded form of the globules of the blood in vertebrate animals.
2. The materials of the blood being sup plied by the digestive apparatus, we might judge, all things else being equal, of the per fection of the blood by the perfection of this apparatus. But there is likewise a necessary co-relation between the result of the function, and the aliment; for instance, when the ap paratus shall be found nearly alike in any two cases, the difference of food necessarily in fluencing the qualities of the blood, the com parison must be established, every other cir cumstance being equal, according to the higher or lower nutritive qualities of the food.
As the use of the arterial blood is to excite and nourish the different parts of the body, there will be a necessary correspondence be tween the blood and the result of the nutrition which may become manifest in the nature and quality of the tissues. And in this case it would be fair to make use of these characters of tissues to form an estimate of the nature of the blood in reference to its aptitude to pro duce heat ; and this we shall accordingly do. But even in the event of all these characters failing us, there is another source whence we can derive comparative measurements, which are susceptible of very rigorous application.