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CIRCULATION IN MAMMALS AND BIRDS.

obstruction exists. Thus, when the admission of air to the lungs is prevented, the blood will not pass through the pulmonary capillaries, since it cannot undergo the change which ought to be performed there; and it therefore accumulates in the pulmonary artery, the right side of the heart, and venous system; and if no relief be afforded by the admission of air into the lungs, the whole circulation is thus brought to a stand. This condition, which is termed Asphyxia, occurs in drowning, hanging, and other forms of suffocation (§ 338).

Course of the Blood in the different Classes of Animals.

281. The Circulation of the Blood takes place on the same general plan in all other MAMMALS, and in BIRDS, as in MAN. In all the animals included in these groups, the heart is composed of two halves quite distinct from each other; each possessing an auricle or receiving cavity, and a ventricle or propelling cavity. The course of their blood, which goes through a complete double circulation, is shown by the diagram (fig. 129). The vessels and cavities of the heart which contain venous blood are shaded; whilst those which convey arterial blood are left white: and this distinction is kept-up in the other figures. The direction of the blood is indicated by the arrows. Every drop of blood which has passed through the capillaries of the system, is transmitted to the lungs before it is allowed again to enter the aorta; and the whole mass of the blood passes twice through the heart, before any part of it is transmitted a second time to the vessels from which it was before returned.

282. The two sides of the heart do not possess, when that organ is perfectly formed, any communication with each other, except through the pulmonary vessels; and thus they might be regarded as two distinct organs, united for the sake of convenience. The right side of the heart, being placed at the origin of the pulmonary artery, and having for its office to propel the blood through the lungs so as to receive the influence of the air, may be called the respiratory heart: whilst the left side, which is placed at the origin of the aorta, and has to propel the blood to the body in general, may be called the systemic heart. The circulation would be performed precisely in the same manner, if these two organs

Fig. 129.-DIAGRAM OF THE CIRCULATION IN MAMMALS AND BIRDS.

were quite distinct from each other; and in fact they are almost so in the Dugong, one of the herbivorous Whales

R

Fig. 131.-DIAGRAM OF THE CIRCULATION IN FISHES.

(ZOOLOGY § 305). In the lower tribes of animals we shall

CIRCULATION IN FETUS AND IN REPTILES.

243

presently find that there is but a single, instead of a double, heart; and that the organ which is absent is sometimes the systemic, and sometimes the respiratory heart.

283. Previously to birth, when the lungs are not yet distended with air, and the aeration of the blood is provided-for in other ways, the circulation takes place on a different plan from that on which it is afterwards performed. There exists at that period an opening in the partition between the two auricles, by which they have a free communication; and there is also a large trunk which passes from the right ventricle into the aorta. By these channels, the blood which is received from the systemic veins can pass at once into the aorta, without going through the pulmonary vessels. But when the young animal begins to breathe, these communications are speedily obliterated; the blood is transmitted through the pulmonary vessels to the lungs; and the whole circulation takes place upon the plan just described. There are occasional instances, however, in which the communication between the auricles remains open, so that the double circulation is never perfectly established; for a portion of the blood is allowed to pass from the right to the left side of the heart, without being aerated in the lungs, so that the blood which is sent to the system contains a mixture of venous with the proper arterial fluid, a state which will be presently seen to be natural in the Reptile. Such cases are recognised by the blueness of the skin, the lividity of the lips, and the indisposition to bodily or mental exertion. Persons affected with this malformation seldom reach adult age.

284. In the class of REPTILES, there is not a complete double circulation; for a mixture of arterial and venous blood is sent alike to the lungs and to the general system; and no part is supplied with the pure arterialized fluid. In general the heart contains only three cavities,-two auricles and one ventricle (fig. 133). One of the auricles receives the venous blood from the system; whilst the other receives the arterialized blood from the lungs. Both these pour their contents into the same ventricle, where they are mingled together; and this mingled blood is transmitted, by the contraction of the ventricle, partly into the lungs, and partly into the aorta (fig. 130). In some Reptiles there is a partial division of the ventricle, so that the mixture of the arterial and venous

244

CIRCULATION IN REPTILES.

blood is not complete; and whilst the blood transmitted to the lungs is chiefly that which has returned from the systemic veins, the blood which enters the aorta for the supply of the

system is chiefly that which has returned from the lungs in an arterialized state. Hence such animals have a circulation which approaches very closely to that of Mammals and Birds; and it is among them that we find the greatest vigour and activity in this generally inert and sluggish class.

285. The general arrangement of the blood-vessels in Reptiles is shown in fig. 134. It is seen that the aorta, soon after its origin, divides into three arches on either side; and that these, after sending off branches to the head and to the lungs, reunite into a single trunk, which corresponds exactly with the aorta of the higher animals. These arches are in fact the remains of a set of vessels, which will be found to be of the highest importance in Fishes, being there subservient to the aeration of the blood: in the true Reptiles, however, they are never concerned in this function, but they still remain, as if to show the unity of the plan on which this apparatus is formed. Precisely the same arrangement of the vessels may be seen in Birds and Mammalia, at an early stage of their development; but it afterwards undergoes considerable changes, by the obliteration of several of the arches; for of the four pairs which may be seen at one period, a single branch only remains on either side; and one of these becomes the permanent arch of the aorta, whilst the other becomes the permanent pulmonary artery.