838 ZOOLOGY. 
no glands are found, either mesenteric or lymphatic, and 
the common duct is double. In birds there are no mesen¬ 
teric, but some few lymphatic glands, though they are not 
strikingly apparent. It is in the mammalia that this system 
of vessels becomes most strongly marked ; yet it has varieties. 
In the sea-otter, the chyle, after being collected in a recep¬ 
tacle, pours itself into the blood by a thoracic duct, formed 
of several branches; and in the order ferae the mesenteric 
glands form one mass, called, from its discoverer, the pan¬ 
creas Ascllii, 
Of the circulating and breathing systems. —The lowest 
of the class vermes, namely, the zoophytes, betray, as we have 
before stated, no traces of a distinct circulating system. The 
Crustacea, and the majority of mollusca, have a heart formed 
of an auricle and ventricle, or of a ventricle only. The ven¬ 
tricle drives the blood into a central aorta, and the vena cava, 
ere it returns it into the heart, ramifies through the gills. The 
heart in the cuttle-fish consists of one ventricle and two 
auricles, which lie at some distance from the ventricle, near 
the gills, or, according to Cuvier, the cuttle-fish has three 
hearts, neither of which possesses an auricle. Two of these 
organs are placed at the root of the two branchiae; they re¬ 
ceive the blood from the body, (the vena cava dividing into 
two branches, one for each lateral heart,) and propel it into 
the branchiae. The returning veins open into the middle 
heart; from which the aorta proceeds. Some bivalves are 
said by Poli to have two auricles, and some even four. But 
in all these crustaceous animals, there has been no connexion 
hitherto discovered between the arteries and veins; orders of 
this class have a connected system of vessels without a heart. 
The heart in fishes is extremely small in proportion to the 
body. Its structure is very simple, as it consists of a single 
auricle and ventricle, which correspond with the right side 
of the heart in warm-blooded animals. The ventricle gives 
rise to a single arterial trunk, (which is expanded in most 
fishes into a kind of bulb as it leaves the heart,) going straight 
forwards to the branchiae, or organs of respiration. The 
blood passes from these into a large artery, analogous to the 
aorta, which goes along the spine and supplies the body of 
the animal; it is then returned by the venae cavae into the 
auricle, which is an arrangement precisely opposite to that of 
the Crustacea, in which the blood traverses the gills as it is 
returning to the heart instead of permeating them as it pro¬ 
ceeds from that cavity. 
The circulating system is the most obscure in the insecta. 
The tracheae, or air-vessels, are plainly visible, but Cuvier 
has failed to detect, with the microscope, either blood-vessels 
or absorbents. The first appearance of an attempt at a 
central circulation is in insects, a membranous tube running 
down the back and performing alternate contractions and 
dilatations. These movements probably effect no more than 
mixing intimately the air that is brought by the trachea with 
the blood, for no vessels can be seen passing from the tube. 
Blumenbach $ays that the frogs, lizards, and serpents of 
Germany have a simple heart, consisting of a single ventricle 
and auricle. That in others, as for instance crocodiles and 
lizards, properly so called, and serpents, the heart consists of 
one ventricle with two auricles. 
But Cuvier describes and del ineates the heart of the cro¬ 
codile, as being formed nearly like that of the turtle; he says 
that the iguana has a similar structure, and that it obtains 
likewise in the serpents. He does not mention the more sim¬ 
ple form as existing in any lizard or serpent. 
The heart of the turtle possesses two auricles, which are 
separated by a complete septum, like those of warm-blooded 
animals, and receive their blood in the same manner as in 
those animals; viz., the two venae cavae terminate in the 
right auricle, the pulmonary veins in the left. Each pours 
its blood into the corresponding ventricle, of which cavities 
there are two; thus the structure of the heart hitherto resem¬ 
bles that of mammalia. 
Three characteristic peculiarities distinguish the heart of 
Ihese animals. First, the two ventricles (and in some species 
of turtles, the cavities of the auricles) are extremely small and 
narrow, but the fleshy walls of this viscus are of a thick and 
spongy texture, so that the heart has the appearance not st? 
much of a double visceral sac, as of a sponge soaked with 
blood. Secondly, both the ventricles communicate with each 
other; 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. Thirdly, the large arterial trunks arise 
all together from the right ventricle only; no vessel coming 
from the left. The aorta, with its three principal branches, 
is situated towards the right side and the upper part; the 
pulmonary artery comes as it were from a particular dilata¬ 
tion of the right ventricle, which is not situated nearly in the 
middle of the basis of the heart. 
Mery and Morgagni considered this dilatation as a third 
ventricle, ventriculus intermedins; hence it has happened, 
that some zootomists have ascribed to the turtle a single ven- 
tricle, (on account of the communication); some two, and 
others three. 
We can now comprehend how this wonderful and anoma¬ 
lous structure, by which all the blood is propelled from the 
right ventricle only, is accommodated to the peculiar way 
of life of the animal, which subjects it frequently to remain¬ 
ing for a long time under water. For the greater circulation 
is so far independent of that which goes through the lungs, 
that it can proceed while the animal is under water, and 
thereby prevented from respiring, although the latter is im¬ 
peded. In warm-blooded animals, on the contrary, no 
blood can enter the aorta, which has not previously passed 
through the lungs into the left ventricle; and hence an ob¬ 
struction of respiration most immediately influences the 
greater circulation. 
The heart of the mammalia and birds differs in no essential 
respect from that of man. All birds have the right ventricle 
very strong and of a triangular shape, which some have 
thought is for the purpose of propelling the blood with great 
force into the lungs of these animals, because they are desti¬ 
tute of that full expansion which the mode of respiration in 
the mammalia affords, and is so favorable to the transmis¬ 
sion of blood. The mammalia, generally, have not the heart 
so close to the diaphragm as man has; the simiae approach 
him most nearly in this respect. 
The heart of the orang-outang is placed obliquely, like 
that of the human subject; but in other simiae the apex only 
is a little inclined to the left, and just touches the dia¬ 
phragm. 
The larger adult bisulca and the pig, have two small flat 
bones, (which have been called, particularly in the stag, 
bones of the heart) where the aorta arises from the left ven¬ 
tricle. 
The right auricle receives, in the porcupine and elephant, 
two anterior venae cavae; the left of which opens near the 
communication with the ventricle. 
We subjoin an account of the beautiful structure of the 
perfect heart, in the class mammalia, as extracted from Sir 
E. Home’s Lectures:— 
“ In considering the muscular structure of the heart, it is 
only intended, at present, to examine the ventricles, which 
may be reckoned two separate muscles; the right ventricle 
for sending the blood .through the vessels of the lungs, called 
the lesser circulation; the left, to propel it through the 
branches of the aorta, which go to every part of the body, 
called the greater circulation. If these two ventricles are 
superficially examined, the muscular fibres by which they 
are united, seem to belong equally to both; one half appear¬ 
ing to be a portion of the right, the other of the left ventricle. 
Were it so, the sides of the left ventricle, although evidently 
more muscular and thicker than those of the right, would by- 
no means be so strong in proportion to the effects they have 
to produce, as it w'ould be natural to expect. We find, how¬ 
ever, upon dissection, that the septum is almost wholly a 
portion of the left ventricle, which gives it a great superio¬ 
rity over the right, and makes it capable of performing the 
important office of supplying the body with blood. The 
muscular structure of the left ventricle, detached from the 
other parts, is an oviform hollow muscle, but more pointed 
at its apex than the small end of a common egg. It is made 
up 
