' 4 10 
human anatomy; but, on the contrary, while 
}■ embraces the whole circle of animated ex- 
istence, considers man as the standard of its 
Comparisons, and the primary object of its 
enquiries. 
] o ascertain the differences among animals 
in their several organs, is the proper object of 
comparative anatomy. In treating of this sub- 
ject, therefore, it is our intention to describe, 
in the first place, the most important of those 
varieties ; which will lead to a classification 
or division of animals, founded on the whole 
of their organization, those being- connected 
in one class which have the nearest resem- 
blance to each other in anatomical charac- 
ters ■, we shall then select two individuals of 
the first classes for the more particular de- 
scription of the internal organs, especially of 
the thoracic and abdominal viscera ; and con- 
clude by taking a general survey; first of 
the structure of animal bodies, as it relates 
to their loco-motive powers ; and secondly, 
in reference to the external form and rela- 
tive quantity of brain or encephalon, as con- 
nected with a gradation of the sentient prin- 
ciple and intellectual faculties. 
General differences which animals exhibit in 
their organs cf sense and motion. 
As the principal characteristics of animal 
life are sensation and loco-motion, we are na- 
turally led to enquire into the differences of 
those organs in which the former faculty is 
found to reside, and likewise into those by 
which the latter is exerted, for the purpose of 
establishing a general and obvious .classifica- 
tion of the objects of comparative anatomy : 
accordingly we find the variations of the in- 
ternal part of the nervous system to present 
three well-marked divisions; the first of 
which, commencing with the lowest order of 
animals, comprehends those in which neither 
vessels nor nerves have hitherto been dis- 
covered ; such are the zoophytes, or polypi, 
which perhaps may be regarded as preferring 
but an equivocal claim to the denomination 
of animals. The second division consists of 
those in which there is only the brain above 
the alimentary canal, while the remaining 
portion of the common bundle of nerves is 
situated beneath it, and is contained in the 
same cavity with the other viscera : these 
form the class of mollusca, crustacea, in- 
sects, and some of the articulated worms. 
The third division comprehends animals 
in which th common fasciculus of nerves is 
placed entirely in the back, and above the 
alimentary tube, being inclosed within a ca- 
nal which passes through the vertebral co- 
lumn: these are the vertebral animals, in 
which* man is included. 
With respect to the external senses, we 
shall find the difference to consist in the 
purnber, and degree of energy, which apper- 
tain to each. 
Ail vertebral animals possess the same 
senses as man. 
Sight appears to be wanting in the zoo- 
phytes, in several kinds of articulated worms, 
in many of the insects, and in the acephalous 
mollusca. The organ of hearing has not 
been discovered in some of the mollusca and 
insects. The remaining three senses, but 
particularly those of taste and touch, appear 
in no instance to be wanting. 
The organs of motion present likewise the 
three following important and primary dis- 
COMPARATIVE ANATOMY. 
tinctions. 1st. The bones either form an in- 
ternal skeleton, articulated, and covered by 
muscles; or, 2dly, these last are placed ex- 
ternally, and enveloped by scales or shells: 
in the" last division there is not any hard part 
that can serve as a lever or fulcrum for mo- 
tion. 
Animals of the first kind have the whole 
body supported by a bony column formed of 
distinct pieces, which is called the spine, ver- 
tebral column, or back-bonc: -these, there- 
fore, are the vertebral animals. 
The animals destitute of vertebra-, accord- 
ing as they are entirely soft, have their bo- 
dies covered with scales, or are enclo. . d in 
shells, belong to the classes of soft worms, in- 
sects, or crustacea. 
Besides, however, tire leading distinctions 
above pointed out from varieties in animal 
organization and functions, we shall find im- 
portant differences likewise in those t/iat are 
called vital and natural functions, and in the 
organs appropriated to each. 
Varieties in the organs of digestion and as- 
similation. 
The organs of digestion afford two leading 
differences in their general dispositions. In 
the major part of the zoophytes the intestines 
form a sac, which has but a single aperture, 
serving at once for the entrance of the ali- 
ment and discharge of the excrement : all 
other animals have two distinct openings for 
these purposes (mouth and anus), placed at 
the two extremities of what is called the ali- 
m*ntary canal. 
The chyle, formed by the action of the di- 
gestive organs, is transmitted to the several 
parts of the body in two different ways, It 
either simply transudes through the sides of 
the alimentary canal, thus in a manner bath- 
ing all the interior of tire body, as in the zoo- 
phytes, and perhaps in insects ; or it is re- 
ceived bv a distinct system of vessels, through 
the medium of which it is conveyed into the 
mass of blood. Some of those animals that 
have absorbent vessels, as man, and other of 
the vertebral class, have the contents of these 
vessels white, while the blood is red ; in 
others the chyle, lymph, and blood, are of 
the same colour. 
Differences in the organs of circulation , re- 
spiration, and voice. 
The circulation of the blood itself is ac- 
companied with very considerable differ- 
ences. 
In the first place, there are animals, viz. the 
insects and zoophytes, which have no- circula- 
tion ; others have a single, and others a double 
circulation. The reptiles afford an example of 
a single circulation ; while in man, indeed in 
ail the mammalia, as likewise in birds, fishes,, 
and many of the mollusca, it is double. 
Where the circulation is single, a great 
part of the venous blood re-enters the arte- 
ries, without passing through the lungs: in 
this manner the blood is distributed in the 
amphibia. Here there is only one heart ; 
when the circulation is double, there .re some- 
times one heart at the base of the pulmo- 
nary artery, and another at the base of the 
aorta, or principal artery of the body ; and 
these two hearts are either united, as in man, 
some other of the mammalia, and birds ; or 
they are separate, and placed at a distance, 
as in the genus sepia, or cuttle-fish. 
Where there is only one ventricle, it is 
[ sometimes placed at the base of the aorta, as 
; in snails and other molluscula, or at the root 
| of the pulmonary artery, as in fishes'. 
1 Great varieties are also observable in the 
i organs subservient to respiration. When the 
i element that acts on the blood is atmospheric 
| air, it penetrates the interior of -the respira- 
1 tory organ; while it simply glides over a 
: multiplied surface, when that element is wa- 
| ter: in this latter case the lamella? of which 
| die organ is composed are called gills, or 
j branchiae ; these are common to fishes, and 
: many of the mollusca ; in some instances, in 
| place of branchial, we meet with fringes, or 
tufts. 
The air penetrates into the interior of the- 
body, either by a single aperture, or by se- j 
veral : the former is observed in all animals 
which have lungs, properly so called. When 
there are several apertures, whfoh is the case 
only in insects, the vessels that receive the j 
air are ramified ad infinitum, and convey it j 
to every part of the body. This last is called ' 
respiration by trachea:. 
Lastly, scarcely any of the zoophytes have j 
perceptible organs of respiration. 
The organs of voice present but two ge- 
neral differences. In birds the glottis is si- 
tuated at the inferior, in quadrupeds and rep- j 
tiles it is placed at the superior, extremity of 
the trachea, and at the root of the tongue. It 
is only the above three classes of animals that 
are furnished with a glottis. Sounds are pro- 
duced in other animals by different means, j 
Sometimes the friction of certain elastic parts 
is employed for this purpose ; sometimes the , 
animal beats the air with other parts, or oc- 
casions a rapid motion in certain portions of I 
that air which is retained in its body. 
V arieties in the generative organs. 
Generation essentially differs both in its 
mechanism and produce. This function is 
performed in some of the zoophytes without 
copulation ; and the young animal grows on 
the body of the parent like the shoots of vege- 
tation. In other animals the sexual organs j 
are either found united in the same indivi- 1 
dual, as in tiie mollusca, and some of the zoo- I 
phytes, qr are placed in two separate indivi- 1 
duals, as in insects, and all the vertebral class 
of animals. 
Some hermaphrodites, as the bivate shell 
fish, generate singly; in others of this class a ' 
conjunction of two individuals is absolutely 
necessary for the production of offspring"; 
each individual performing, at the same time, ' 
the function of both male and female: this is 
the case with snails, and other species of mol- 
lusca that crawl upon the belly. 
The product of generation is either a bud 
projecting from the parent like the branch of : 
a tree, till completely developed ; or a foetus, 
which arrives at maturity in the womb of its 
mother, and from which it issues a living ani- 
mal; or, finally, it is an embryo, inclosed in 
a shell, with a substance adhering to, and 
connected with, it by vessels; which sub- 
stance it absorbs and converts into nutriment 
before it is completely hatched. The above 
varieties form die three modes of generation 
denominated gemmiparous, viviparous, and 
oviparous. The first is confined to a few zoo- ! 
phytes and articulated worms; the second 
takes place in man and other mammalia only ; 
the third is common to all other animals, 
with the partial exception of those which, 
