COMPARATIVE ANATOMY. 
411 
although in reality oviparous, are actually 
generated living animals, on account of the 
eggs being hatched in the oviduct of the mo- 
ther. This is the case for instance, with the 
viper. Some animals have at birth the same 
form which they afterwards retain. Others 
vary considerably in then- perfect state from 
the form which they received at birth, and are 
said therefore to undergo a metamorphosis. 
Hitherto this change has only been observed 
in insects, and in reptiles without scales, that 
is, in frogs and lizards. 
Organs of secretion. 
The organs of secretion likewise vary ma- 
terially in diiferent animals : in the zoophytes 
secretory vessels have not been detected; 
in insects secretion is effected by tubes of 
different length ; and in the vertebral animals, 
as likewise in some of the mollusca, this func- 
tion is performed by glands, or at least vas- 
cular expansions. 
Classification of Animals, founded 
ON THE WHOLE OF THEIR 
ORGANIZATION. 
The comparative anatomist divides the 
■whole animal kingdom into two great fami- 
lies: the lirst comprehends those animals 
which have vertebras and red blood: the se- 
cond is composed of those which are desti- 
tute of vertebras, and which, for the most 
part, have white blood. 
The general anatomy of the first class is 
the following. 
Anatomy of vertebral animals.with red blood. 
The skeleton is invariably covered by 
fleshy substance, and is articulated, its prin- 
cipal support is the vertebral column, which 
always joins the head at its anterior extre- 
mity', and is frequently prolonged at its pos- 
terior extremity to form the tail. The ribs, 
which are constantly met with in animals of 
this class, are attached to both sides of the 
spine. There are never more than four 
limbs ; two, or all of which, are sometimes 
wanting. The brain is always contained in 
the cranium ; the senses are* invariably five 
in number; the eyes are two; the ear has at 
least three semicircular canals ; the sense of 
smell has always its seat in the cavities at the 
anterior part of the head. 
The circulation is maintained by a heart 
consisting of at least one ventricle : if there 
are two, these are never separate. The 
absorbent is distinct from the venous system. 
The two jaws are situated horizontally: 
the mouth always opens by their separation 
from above downwards. 1 he intestinal ca- 
nal is continued from the mouth to the anus, 
which is uniformly found at the posterior 
part of the pelvis. r l he intestines are sur- 
rounded by a peritoneum. A liver, pancreas, 
and spleen, are always met with. 
The kidneys are placed on each side the 
spine and without the peritoneum. ihe 
testicles are tw 7 o in number. Above the 
kidneys two bodies are found, the use of 
which has not hitherto been discovered. 
This lirst class is again subdivided into two 
branches, consisting 1st of animals with warm 
blood, and 2dly of animals with cold blood. 
Vertebral animals with warm blood. 
The first of these have invariably two 
ventricles and a double circulation. Animals 
of this class cannot exist without respiration. 
Their brain occupies the whole cavity ot the 
cranium. Their eyes close by palpebral. The 
tympanum of the ear is sunk in the scull. 
The labyrinth of this organ is enveloped 
by the bone ; and there is always observed in 
it, besides the semicircular canal, an organ re- 
sembling the shell of a snail (cochlea). Their 
nostrils have a communication with the back 
of the mouth. Their trunk is invariably sur- 
rounded with ribs; the greater number of 
this class of animals have four limbs. 
With cold blood. 
The second subdivision of this first class, 
the vertebral animals with cold blood, are fre- 
quently . without ribs ; some have no articu- 
lated limbs. Their brain never tills up the 
whole cavity of the cranium, h ire eyes are 
seldom furnished with moveable palpebrs. 
When there is a tympanum to the organ of 
hearing, it Ls always level with the head. 
The cochlea is wanting. The various parts 
of the ear are not attached closely to the 
cranium; sometimes they are at liberty in 
the same cavity w ith the brain. 
Each of these two branches of vertebral 
animals, is again subdivided into two classes; 
the two divisions of the lirst branch arc the 
mammalia and birds. 
First class of vertebral animals with warm 
. blood — The Mammalia. 
The mammalia are viviparous : they nou- 
rish their young with milk which is furnished 
by their mammae ; they have always an 
uterus with two cornua ; the males have a 
penis, which in copulation is introduced 
mto the genital organ of the female. 
The head of the mammalia is joined to the 
first vertebra by two eminences : the cervical 
vertebrae are never less than six, nor more 
than nine ; the brain is extremely compli- 
cated : it has parts which are not met with in 
other classes of animals, such as the corpus 
callosum, fornix, pons Varolii, &c. See 
Anatomy. 
The eyes of the mammalia have only two 
palpebra? ; their ears have four articulated 
bones, and a spiral cochlea ; their tongue is 
soft and fleshy : the skin is in a great number 
of instances covered w r ith hair, and there 
is some hair on all of them. 
The lungs are enclosed within the thorax : 
there is always a diaphragm : there is only 
one larynx, which is situated at the base of 
the tongue, and covered by the glottis during 
deglutition. 
The inferior maxilla only is moveable ; 
both jaws are furnished with lips. 
The biliary and pancreatic ducts are in- 
serted at the same point : the lacteal vessels 
contain a milky chyle, and pass through the 
mesenteric glands. The epiploon covers 
the anterior portion of the intestines. 
The spleen is invariably situated on the 
left side ; between the stomach, ribs, and 
diaphragm. 
Second class — Aves.- 
Birds are oviparous ; they have but one 
ovarium and one oviduct, a structure pecu- 
liar to this class. The head is connected 
with the vertebrae by one eminence only : 
the vertebrae of the neck are very numerous. 
The sternum is much expanded: the anterior 
members are only used in flying, the posterior 
in walking. 
The eyes of birds have three palpebrae ; 
they have dq external ear : the tympanum 
3 FJ 
has only one hone : the cochlea is in the 
form of a cone slightly tw’isted ; the tongue 
has a bone internally ; the body is covered 
witli feathers ; the lungs are attached to the 
ribs ; in the act of inspiration air is com- 
municated to all the body 7 , as birds have no 
diaphragm. The windpipe has a larynx at 
each extremity; the upper extremity is with- 
out an epiglottis. Their mouth is formed by 
a bill or beak of a horny consistence, the two 
mandibles of which are moveable. 
Several ducts pass out from the pancreas 
and liver, and penetrate the intestinal canal 
at different parts. The chyle is transparent, 
and there are neither mesenteric glands nor 
omentum. The spleen is in the centre of 
the mesentery, 
The urinary ducts (ureters) terminate in 
a cavity called the cloaca, which is the re- 
ceptacle both of the excrements and eggs. 
Birds have no urinary bladder. 
General anatomy of animals with cold blood 
belonging to the vertebral class ; these are , 
reptiles and fishes : third class of animals — 
Reptiles. 
There is less uniformity in the organization 
of reptiles thdn in that of any other class ; 
some walk, some fly, others swim, and many 
can merely crawl. Their organs of sensation, 
particularly the ear, exhibit nearly as many 
varieties as those of motion. Reptiles are 
however universally without cochlea; their 
skin is either naked, or enveloped with a 
scaly covering: their brain is invariably 
small ; the lungs are contained in the same 
cavity with the other viscera, but they do 
not admit the air to pass beyond them. The 
pulmonary cells are very capacious ; there is 
only one larynx which has an epiglottis : both 
the jaws are moveable ; they are without 
omentum or mensenteric glands ; the spleen, 
as in birds, is in the centre of the mesentery. 
The female has two ovaries and two oviducts ; 
reptiles have an urinary bladder. 
Fourth class — Pisces. 
Fishes respire by organs in the shape of 
gills situated on either side of the neck, be- 
tween which they transmit the water ; they 
have therefore no trachea, larynx, nor voice; 
their bodies are formed for swimming, but 
they have sometimes no fins. Besides the 
four which represent the extremities, they 
have vertical fins on the back, under the tail, 
and at its extremity. 
Their nostrils do not assist them in respi- 
ration: their ear is concealed within the 
cranium : their skin is either without any 
covering, or is scaly ; their tongue is bony ; 
each jaw is moveable ; the intestinula coeca 
often occupy the place of the pancreas ; they 
have an urinary bladder : their ovaries are 
double. 
General anatomy of animals without vertebrae. 
The organization of the second family of 
animals, the invertebral, presents a less re- 
gular series to assist the comparative anat- 
omist in his classification. If animals of this 
class have an articulated and hard skeleton, 
it is for the most part external. Their nerv- 
ous system, instead of being enclosed in a bony 
case, floats in the same cavity with the other 
viscera. The brain furnishes two branched 
which envelope the oesophagus ; the con- 
tinuation of these branches forms the re- 
mainder of the common mass of nerve*. 
