Three forms of heart are found in the Sauropsida
first is that observed in the Chelonia
, and Ophidia;
the second, that in the Crocodilia
; and the third, that
1. In the Chelonia, Lacertilia,
two auricles. Generally, a distinct sinus venosus,
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.
|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
No arterial trunk arises from the cavum arteriosum
two arterial trunks arise from the right-hand end of the cavum
; these are the two aortic arches. One of these
passes to the left and the other to the right side, and they
cross one another as they do so, because the origin of the left
arch lies more to the right than does the origin of the right
arch. The ostia of both arches are guarded by semi-lunar
valves; and that of the left arch is placed below and to the
right of that of the right arch. As no arterial trunk arises
from the cavum arteriosum
, the red blood can be driven out
of the latter, during the systole, only into the cavum venosum.
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
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
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.