Rort of trinsverse bony fibres : tlie broad 
bones are filled with a reticulated bony 
texture, the cells of which are empty. 
They have considerable apertures, (most 
easily .shewn in those extremities of the 
cylindrical bones which are tin ned towards 
the sternum) communicating with the lungs 
by small air-cells; which facts may be 
shewn by various experiments on living 
and dead birds. 
These receptacles of air probably serve 
the purpose of lightening the body of the 
bii-d in order to facilitate its motions. This 
effect is produced in most birds to assist 
their flight; in some aquatic species, for 
the purpose of swimming; in the ostrich 
and some otheis, for running. Hence we 
find the largest and most numerous bony 
cells in birds which have the highest and 
most rapid flight, as the eagle, &c. And 
hence also the bones of the bird which 
has just left the egg, are filled with a 
bloody marrowq which is absorbed soon 
after birth, entirely in some, in others, par- 
ticularly among the aquatic species, at 
least for the greater part. 
Besides the uses which have been already 
pointed out, these receptacles of air dimi- 
nish the necessity of breathing frequently 
in the rapid and long continued motions 
of several birds, and in the great vocal ex- 
ertions of the singing birds. 
The lungs of amphibia are distinguished 
from those of warm-blooded animals, both 
by a great superiority in point of size, as 
well as by a greater looseness of texture ; 
arising from the great size of tlieir air-vesi- 
cles. In frogs, lizards, and serpents, the 
lungs consists of a cavity, whose sides are 
pellular. The posterior part of the organ 
either forms a mere membranous bag, or 
else the cells are larger there than "else- 
where. In the turtle the vesicles are very 
large, but the texture is uniform through- 
out, 
In the tadpole, and the young of such 
lizards as bring forth in w'ater, there are 
two, organs, which somewhat resemble the 
gills of a fish (appendices fimbriatae Swam- 
merdam.) These serve for the purposes 
of respiration while the animal lives in the 
water. They are connected to the sides 
pf the neck, and hang loose from the ani- 
mal ; they are not permanent, but are grar 
dually witlidrawn into the chest, (within 
a few days, in the reptiles of this country,) 
yvliere their remains may still be perceived 
for some time near to the true lungs. In- 
stead of the branchial opening, by whiph 
E ANATOMY, 
fishes again discharge the water which they 
have taken in at the mouth, some tadpoles 
have for this purpose a canal on the left 
side of the head near the eye ; which must 
be distinguished from the small tube on the 
lower lip, by which they attach themselves 
to aquatic plants. 
Instead of lungs, fishes are furnished 
with gills or branchiae ; which are placed 
behind the head, on both sides, and have a 
moveable gill cover (operculum branchiale,) 
which is wanting in the order of pisces 
chondropterygii only. By means of these 
organs, which are connected witli the throat, 
the animal receives its oxygen from the 
air contained in the water; as tliose ani- 
mals which breathe, derive it immediately 
from the atmosphere. They afterwards 
discharge the water through the branchial 
openings (apertuim branchiales); and there- 
fore they are distinguished from animals 
.of the three preceding classes by this cir- 
cumstance ; viz. that they do not resjiire by 
the same way that they inspire. ^ 
We have already shewn in speaking of 
the organs of circulation, how the gills re- 
ceive the venous blood by means of the 
branchial artery, and how this blood is 
sent into the aorta after its conversion into 
the arterial state. The distribution of 
these vessels on the folds and divisions of 
the gills constitutes one of the most deli- 
cate and minute pieces of structure in the 
animal economy. 
Each of the gills consists in most fishes 
of four divisions, resting on the same num- 
ber of arched portions of bone or cartilage, 
connecteif to the os hyoides. Generatly 
there is only a single opening for the dis- 
charge of the water ; but in many cases, 
particularly among the cartilaginous fishes, 
there are several. 
Many animals of this order possess a 
single or double swimming bladder ; which 
has been found in different instances to 
contain azote, hydrogen, and oxygen. It 
has not been hitherto determined, whe- 
ther it be subservient to any other functions 
besides that well known one from which its 
name is derived. In the mean time, like 
the air-receptacles of birds, it may be con- 
sidered w’lthout impropriety in the present 
division of the work. 
It is placed in the abdomen, and closely 
attached to the $pine. It communicates 
generally with the (esophagus, and some- 
times with the stomach, by a canal (ductus 
pneumaticus,) containing in some instances, 
as the carp, valves which seem to allow thq 
