ORGANS OF CIRCULATION.
A perfect circulating system, to which, on the one hand, fluids are brought by the absorbents to be converted into blood; and from which, on the other side, vari ous juices are separated in glands, and viscera of a glandular structure, appears to belong universally and exclusively to red-blooded animals. A pericardium ex ists in all these animals. Parts of such a system, particularly a heart, and cer tain vessels connected with it, are found in some genera of the white blooded classes.
It has been supposed that the amphi bious animals of this class, and the ceta cea, have an open foramen ovale, like that of the foetus, in their septum auricula rum. And the necessity of such an open ing has been inferred from their way of life, since they often pass a considerable time under water without breathing. This supposition has been fully refuted by the repeated dissection of adult animals of this kind ; which has shewn that an ex ception from the general rule very rarely occurs.
In several genera and species of web footed mammalia, and cetacea (that is, in the common and sea-otters, in the dol phin, &c.) particular vessels have been observed to be considerably and con stantly enlarged and tortuous. This struc ture has been principally remarked in the inferior vena cava ; where there can be no doubt that it serves, while the animal is under water, to receive a part of the returning blood, and to retain it until respiration can be again performed, and the lesser circulation be thereby again put in action.
There are some remarkable circum stances in the distribution of particular arteries in certain animals of this class. We may notice, as the most singular of these, the rete mirabile, formed by the internal carotid at its entrance into the cranium, in several ruminating biscula, and carnivorous animals ; and that divi sion of the arterial trunks of the extre mities, which has been observed by Mr. Carlisle in the slow-moving animals, viz. the sloths, and lemur tardigradus. The arteries of the arm and thigh, in these cases, divide, as they leave the trunk, into numerous parallel branches, which are united again towards the elbow and knee.
All birds possess a very remarkable peculiarity in the structure of the heart. The right ventricle, instead of having a membranous valve (such as are found in both ventricles of mamnralia, and also in the left of birds,)is provided with a strong, tense, and nearly triangular muscle. This singular structure assists in driving the blood with greater force from the right side of the heart into the lungs ; since the expansion of the latter organs by re spiration, which facilitates the transmis sion of the carbonated blood in mamma ha, does not take place in birds, on ac count of the connection which their lungs have with the numerous air-cells, which will be afterwards described.
Frogs, lizards, and serpents, have a simple heart, consisting of a single ven tricle and auricle.
The structure of this part is very dif ferent in the turtle, and has given rise to more controversy than that of any order of animals. Their heart missesses two auricles, which are separated by a com plete septum, like those of warm blooded animals, and receive their blood in the same manner as in those animals, viz. the two venx caves terminate in the right au ricle, the pulmonary veins in the left. Each pours its blood into the corres ponding ventricle, of which cavities there are two : thus the structure of the heart hitherto resembles that of mammalia, The characteristic peculiarities which distinguish the heart of these animals con sist in two circumstances : first, both the ventricles communicate together ; there is a muscular, and as it were tubular valve, going from the left to the right cavity, by means of which the former opens into the latter. Secondly, the large arterial trunks arise altogether from the right ventricle only, (no vessel coming from the left.) The aorta, forming three grand trunks, is situated towards the right side and the upper part ; the pulmonary artery comes as it were from a particular dilatation, which is not situated in the middle of the basis of the heart, but lower ; (it must be understood that we apply these terms according to the horizontal position of the animal.) We can now comprehend how this won derful and anomalous structure, by which all the blood is propelled from the right ventricle only, is accommodated to the pe culiar way of life of the animal, which sub jects it frequently to remaining for a long time under water. For the greater circu lation is so far independent of that which goes through the lungs, that it can proceed while the animal is under water, and there by prevented from respiring, although the. latter is impeded. In warm blooded ani• mals, on the contrary, no blood can enter the aorta, which has not previously passed through the lungs into the left ventricle ; and hence an obstruction of respiration most immediately influences the greater circulation.