The Heart in Sauropsida
Three forms of heart are found in the Sauropsida. The first is that observed in the Chelonia, Lacertilia, and Ophidia; the second, that in the Crocodilia; and the third, that in Aves.1. In the Chelonia, Lacertilia, and Ophidia, there are two auricles. Generally, a distinct sinus venosus, with contractile walls, and communicating by a valvular aperture with the auricle, receives the blood from the venae cavae, and pours it into the right auricle. The pulmonary veins usually open by a common trunk into the left auricle.
The interauricular septum is rarely (in some Chelonia) perforated. Its ventricular edge spreads out on each side into a broad membranous valve, the edge of which, during the systole, flaps against a ridge, or fold, developed, on one, or both sides, from the margin of the auriculo-ventricular aperture, and constituting a rudiment of a second valve. The ventricle contains only one cavity, but that cavity is imperfectly divided into two or three chambers, by septa developed from its muscular walls.
In the Turtle (Fig. 92), a partly muscular, and partly cartilaginous, septum extends from the front wall of the ventricular cavity toward its right-hand end. It imperfectly divides the common ventricular cavity into a right small, and a left large moiety. The latter of these receives the blood from the auricles. In consequence of the elongated form of the ventricular cavity, and the projection into it of the large auriculo-ventricular valves, especially of that of the right side, this left and larger moiety of the common ventricle is virtually divided into two, a left and a right, at the time of the auricular systole. The left portion becomes filled with arterial blood from the left auricle, and is distinguished as the cavum arteriosum; the right receives the venous blood from the right auricle, and is the cavum venosum.
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| Fig. 92. - Tlie Heart of a Turtle (Chelone midas).-A, a drawing from nature: the ventral
face of the ventricle being laid open. B, a diagram explanatory of the arrangement of
the cavities and vessels. R. A., L. A., right and left auricles, w, x, arrows placed In
the auriculo-ventricular apertures to indicate the course of the blood at the auricular
systole. v, the right, and v1, the left median auriculo-ventricular valves. C. v., cavum
venosum. C. p., cavuw pulmonale, a, the incomplete septum which divides the
cavum pulmonale from the rest of the cavity of the ventricle. P. A,, pulmonary artery. R. Ao., L. Ao. right and left aortae. s, arrow showing the course of the blood in the
left aorta; t, in the right aorta; z, in the pulmonary artery; y, between the cavum
venosum and cavum pulmonale; x,in the left, and w,id the right auricalo-ventricubu aperture. |
The right, comparatively small, moiety of the ventricle is separated from the cavum venosum by the already-mentioned septum, which is attached between the origin of the left aortic arch and that of the pulmonary artery, its free edge looking toward the dorsal face of the heart. Thus the pulmonary artery arises from what is, virtually, a separate subdivision of the ventricle, or a cavum pulmonale.
When the systole of the ventricle takes place, the practical result of these arrangements is, that the pulmonary artery, and the aortic arches, at first, receive wholly venous blood from the cavum venosum and cavum pulmonale. But as the arterial blood of the cavum arteriosum is driven into the cavum venosum, the venous blood of the latter tends to be excluded from the mouths of the aortic arches, and to be driven into the cavum pulmonale, while the aortic arches receive arterialized blood. The left arch receives a larger proportion of venous blood than the right. As the ventricle contracts, the free edge of the muscular septum approaches the dorsal wall of the ventricle, and gradually closes the access to the cavum pulmonale, which thus finally expels the venous blood which it received from the cavum venosum,, but admits none of the arterialized blood; consequently none of this reaches the lungs.
2. In the Crocodilia, the cavum venosum and the cavum arteriosum are converted into perfectly distinct right and left ventricles. The right ventricle gives off the pulmonary artery, and, in addition, an aortic arch which crosses over to the left side. From the left ventricle only a single trunk arises, and this, crossing to the right side, becomes the right aortic arch, of which the dorsal aorta is the direct continuation. The walls of the two aortic arches are in contact where they cross one another; and, at this point, a small aperture, situated above the semilunar valves, places the cavities of the two arches in communication.
Thus, in the Crocodilia, the venous and the arterial currents communicate only outside the heart, not within it as in the foregoing groups.
The septum of the cavum pulmonale remains as a small muscular band, and the fold of the outer lip of each auriculoventricular aperture has become a distinct membranous valve.
3. In Aves, the venous and arterial blood currents communicate only in the pulmonary and systemic capillaries. The auricular and ventricular septa are complete, as in the Crocodilia; but the right ventricle gives off only the pulmonary artery, the left aortic arch having disappeared. The septum of the cavum pulmonale becomes a great muscular fold, and takes on the function of an auriculo-ventricular valve. At the origin of the pulmonary artery, and at that of the aortic arch, three semilunar valves are developed.
In Reptiles there are usually only two aortic arches, one on each side, answering to the fourth pair of arches of the embryo. The right gives off the carotid and subclavian arteries, and passes directly into the trunk of the dorsal aorta. The left commonly gives off visceral arteries, and becomes a good deal diminished in size before joining the common trunk.
In many Lacertilia, four aortic arches (answering to the third and fourth pairs of the embryo) persist, two anterior arches, from which the carotids are given off, springing, by a common trunk, from the right ordinary aortic arch.
In the Reptilia, most of the blood of the hind-limbs and tail passes through one or other of two "portal systems" before reaching the heart, the one portal system lying in the kidney, the other in the liver. The portion which goes to the liver is carried to it mainly by the anterior abdominal veins, which are represented by two trunks in most Reptilia, by one in the Ophidia.
In Aves there is no renal portal system, and the anterior abdominal vein opens into the inferior vena cava close to the heart. Nevertheless a median trunk, which is given off from the caudal vein, carries a considerable proportion of its blood directly into the hepatic portal system.
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