The different media in which animals live involves the supposition of another modifica tion as to the mode in which the blood or nutritive fluid is aerated. Those that live in air respire this elastic fluid immediately; those that live in water, again, respire it mingled with or dissolved in the surrounding medium. The trachece of those animals whose respira tion is diffuse, and that exist on the surface of the earth, consequently are filled with air ; those of the creatures that exist in water are conduits for the constant transmission of this fluid. When the respiration is concentrated, corresponding modifications in the function are encountered according to the medium in which animals live : the air is either received immediately into the body, when the apparatus is known as a lung, or, suspended among water, it is passed over the surface of the respiratory organ, which is then denominated gill. Quadrupeds and birds respire univer sally by means of lungs, fishes and the mol lusca by means of gills. In certain reptiles the function is carried on by means both of lungs and gills, and as it would appear even by the general surface of the body either vica riously, or at one and the same time. These are the only true amphibious animals.
A circulation, properly so called, is the ap panage of an organization already somewhat complicated, consequently of an animal con siderably raised in the scale of creation. This function, it is evident, as implying in its sim plest sense a progressive motion of the general nutritive fluid or blood, can only exist where such a fluid is encountered. It is altogether wanting, therefore, among those animals in which nutrition is accomplished immediately. We ascend but a little way in the scale before we find the function consisting not only of an outward or progressive motion of the nutritive fluids, but of a retrograde motion also of these same fluids modified in their nature, and re quiring exposure to a greater or less degree in some form of respiratory apparatus to fit them anew for distribution to the organization at large. The fluid in this instance parts from a centre, and returns thither after having made the round of the system. Circulation in this acceptation only occurs among those animals that have a separate respiratory apparatus, and in which we meet with absorption of nutri ment from without, and of lymph, &c. from within. The pabulum of nutrition is taken imp by lacteals and veins from the digestive apparatus, and by veins and lymphatics from the rest of the organism for transmission, under the name of venous blood, to the apparatus of respiration, whatever its form. In this the fluid, still immature and unapt for assimilation, is exposed in vessels of infinite minuteness and extreme tenuity to the action of the at mospheric air, and having undergone in these a certain change, it begins to be collected by another set of vessels, which form branches suc cessively of larger and larger size, until finally it is projected from the respiratory apparatus in one or more trunks, under the name of arterial blood, fitted for assimilation by the organization at large, and proving the principal stimulus under the influence of which its various par ticular organs accomplish their offices.
Circulation, however, as a function, is com plicated in the same degree as the apparatus by which it is effected. In some classes we find the circulation taking placing through vessels only, one set distributing the blood from the respiratory apparatus to the body generally, another collecting this fluid again, and the newly-absorbed matters from the body at large, and transmitting these for elaboration anew in the organ of respiration. In other tribes, and
this invariably after the very lowest grades of the scale are passed, we find the hollow muscle, or forcing apparatus, which, in glancing at the differences of structure, we have spoken of as the heart superadded to the circle of vessels, which even in its simplest state consists of at least two cavities communicating with one another, one for the reception of the blood from, the other for the projection of this fluid to the general system.
But the blood does not follow the direct and simple course here supposed in almost any case. There is the aeration of the fluid in the way, and means to accomplish this important end must of course be provided. Among many animals it would appear by no means necessary that the whole of the blood should undergo exposure in the respira tory apparatus, in order to fit it for the wants of the organization ; a part sent thither, and this on admixture with the remainder suffices to revivify the mass. In this case it is not imperative that the two kinds of blood— the unaerated or venous, and the aerated or arterial—should be kept distinct; there is con sequently no occasion for more than one re cipient cavity or auricle, into which the aerated blood from the organ of respiration, and the unaerated blood of the system are poured in common and mingled, and one projecting cavity or ventricle from which the mixed cur rent is distributed partly to the respiratory ap paratus . and partly to the system at large. Here the blood in its course describes no more than a single circle, beginning and ending in the heart, which is then characterized as simple, consisting, as has been said, of a single auricle and a single ventricle. Among other tribes of animals, however, the whole mass of blood requires to undergo aeration in the respiratory apparatus each time it completes its round before it can again subserve the wants of the organization. In this instance it is evident that the aerated and unaerated- blood require to be most particularly prevented from commingling, and that a single or simple heart will no longer suffice as the implement of circulation. This complex circulation is met with among ani mals so low in the scale as to be unprovided with a heart, when of course it is accomplished by means of vessels only. In some tribes the one portion of the function is performed by the medium of vessels, the other by the agency of a heart which is now connected with the gene ral systemic circulation, now with the pul monic, being situated in the one case in the course of the aerated, in the other in that of the unaerated current of blood. In the most elevated classes of animals, finally, the double circulation is effected by means of two hearts, one dedicated to the projection of the un aerated blood into the lungs, the other to the propulsion of the aerated fluid through the general system. These two hearts, indeed, adhere to one another, and are usually spoken of as if they constituted no more than a single organ, having however four cavities, two auricles and two ventricles, but they are not less distinct on that account, and are severally the centre of a particular circula tory system, one of which commencing in the cavities for the venous or unaerated blood, ex tends through the respiratory apparatus (then uniformly a lung), and back to the cavities for the arterial aerated blood ; the other, com mencing in the cavities just named, extends to every part of the organization, and terminates in the cavities for the unaerated blood, where the lesser round recommences, to be followed in its turn by the greater, and so on, during the whole period of existence.