COMPARATIVE ANATOMY. 
ing organ in tlxat class is therefore a pul- 
monary heart. 
According to Cuvier, the cuttle-fish has 
three hearts, neitlier 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. Tiie returning veins open into 
the middle heart; from which the aorta 
proceeds. 
The other mollusca have a simple heart, 
consisting of one auricle and ventricle. 
The vena cava assumes the otfice of an 
artery, and canies the returning blood to 
the gills ; whence it passes to the auricle j 
and is subsequently expelled into the aorta. 
Here therefore, as in the Crustacea, tlie heart 
is a pulmonary one. 
The vermes of Cuvier have circulating 
vessels, in which contraction and dilatation 
are perceptible, without any heart. They 
can be seen very plainly in the lumbricus 
marinus. The leech, naias, nereis, aphro- 
dite, &c. are further examples of the same 
structure. This anatomist is of opinion 
that the mollusca, Crustacea, and vermes, 
possess no absorbing vessels ; and he thinks 
that the veins absorb, as he finds them to 
have communication with the general ca- 
vity of the body, particularly in the cuttle- 
fish. Hence the above mentioned classes 
will hold an intermediate rank between 
the vertebral animals, which possess both 
blood-vessels and absorbents, and the in- 
sects which have neither. 
ABSORBING SYSTEM. 
The chyle of birds is transparent : and 
there are no mesenteric glands in these 
animals. 
The lacteals are uncommonly numerous 
on the intestines, and mesentery of the 
turtle, in which animal there are no absor- 
bent glands. 
The lymphatics of fishes have neither 
glands nor valves. 
ORGANS OF respiration. 
The incessant continuation of the great 
chemical process, by which oxygen is ex- 
changed for hydrogen and carbon, is es- 
sentially necessary to the well being of the 
greater part of animals. Yet the organs 
and mechanism, by which this wonderful 
function is carried on, vary very conside- 
rably. In the mammalia after birth ; in 
birds when they have left the eg| ; and in 
amphibia when completely formed, the 
chief organ of this function is the lungs : 
in fish it is performed in the gills j in most 
insects in their trache® ; in the vermes, in 
analogous, but at the same time very dif- 
ferent parts. 
The respiratory organs of bii-ds consti- 
tute one of the most .singular structures in 
the animal economy, on account of several 
peculiarities which they possess ; but more 
particularly in consequence of their con- 
nection with the numerous air-cells, which 
are expanded over the whole body. 
The lungs themselves are comparatively 
small, flattened, and adhering above to the 
chest, where they seem to be placed in 
the intervals of the ribs; they are only 
covered by the pleura on their under sur- 
face, so that they are in fact on the out- 
side of the cavity of the chest, if we con- 
sider that cavity as being defined by the 
pleura : a great part of the thorax, as well 
as the abdomen is occupied by the mem- 
branous air-cells, into which the lungs open 
by considerable apertures. Those of the 
thorax are divided, at least in the larger 
birds, by membranous transverse septa, 
into smaller portions; each of which, as 
well as the abdominal cells, has a particu- 
lar opening of communication with the air- 
cells of the lungs, and consequently with 
the trachea. The membranes of these cells, 
in the larger birds, are provided here and 
there with considerable fasciculi of muscu- 
lar fibres, which have been regarded as a 
substitute for the diaphragm, which is 
wanting in this class of animals. They 
also serve very principally, as we may as- 
certain by examining large birds in a living 
state, to drive back again into the lungs; 
the air which they received in inspiration ; 
whence the repletion and depletion of the 
thoracic cells must alternate with those of 
the abdominal cavities. 
Besides these cells, a considerable por- 
tion of the skeleton is formed into recepta- 
cles for air in most birds, (for there are 
indeed exceptions and considerable varia- 
tions in the different genera and species.) 
This structure is particularly marked in the 
larger cylindrical bones, as the scapula, 
clavicle and femur. It is also found in most 
of the broad and multangular bones of the 
trunk, as the sternum, ossa innominata, 
dorsal vertebr®, &c. All these are desti- 
tute of marrow in the adult bird, at least 
in their middle; so that the cylindrical 
bones form large tubes, which are only in- 
terrupted towards the extremities by a 
