COM PAR ATI VE ANATOMY. 
formel- ease he wDuUl not be able to look 
before him ; and the great weight of the 
head, with the coilipaiative weakness of the 
■extensor innsclbs, and the want ofligamen- 
tiim nuchae would render the elevation of 
that organ almost impossible. 
When quadrupeds endeavour to support 
themselves on the hind extremities ; as, for 
instance, for the purpose of seizing any ob- 
jects with the fore feet, they rather sit 
down than assume the erect position. For 
they rest on the thighs as well as on the 
feet, and this can only be done where the 
fore part of the body is small, as in the si- 
mi*, the squirrel, &c. ; in other cases, the 
animal is obliged also to support itself by 
the fore feet, as in the dog, cat, &c. The 
large and strong tail in some instances forms 
as it were a third foot, and thereby in- 
creases the surlace for supporting the body, 
as in the kanguroo and the jerboa. 
Various gradations may be observed in 
the mammalia, connecting man to those 
.animals which are strictly quadrupeds. The 
simi*, which are by no means calculated 
for the erect position, are not, on the other 
hand, destined like the proper quadrupeds to 
go on all-fours. They live in trees, where 
their front and hind extremities are both 
employed in climbing, &c. 
The true quadrupeds have the front of 
the trunk supported by the anterior extre- 
mities, which are consequently much larger 
and stronger than in man ; as the hind feet 
of the same animals yield in these respects 
to those of the human subject. The chest 
is in a manner suspended betrveen the sca- 
pul*, and the serrati magni muscles, which 
support it in this position, are consequently 
of great bulk and strength. When viewed 
together they represent a kind of girth sur- 
rounding the chest. 
The chief agents in flying are the muscles, 
■ which move the anterior extremities of the 
bird, and which constitute what in common 
language is termed the breast of the ani- 
mal. 
Birds possess three pectoral mirscles, 
arising chiefly from their enormous sternum, 
and acting on the head of the humerus. 
The first, or great pectoral, weighs of itself 
more than all the other muscles of the bird 
together. The keel of the sternum, the 
ibrk,- and the last ribs, give origin to it ; 
and it is inserted in a rough projecting line 
of the humerus. By depressing that bone, 
•it produces the streng and violent motions 
of the wing, which carry the body forwai-ds 
in, flying, The middle pectoral lies under 
this, and sends its tendon over the junction 
of the fork, with the clavicle and scapula, 
as in a pulley, to be inserted in the upper 
part of the humerus; which bone it ele- 
vates. By this contrivance of the pulley, 
the elevator of the wing is placed at the un- 
der surface of the body. The third, or 
lesser pectoral muscle, has the same eflFect 
with the great pectoral, in depressing the 
wing. 
One of the flexor tendons of the toes of 
birds, (produced from a muscle which 
comes from tlie pubis) runs in front of the 
knee ; and all these tendons go behind the 
heel: hence the flexion of the knee and 
heel produces mechanically a bent state of 
the toes, which may be seen in the dead 
bird ; and it is by means of this structure 
that the bird is supported, when roosting, 
without any muscular action. 
This circumstance of the flexion of the 
toes accompanying that of the other joints 
of the lower extremity of birds, was long 
ago observed by Borelli, and attributed by 
him to the connection which the flexors of 
the toes have with the upper parts of the 
limb, by which they are mechanically 
stretched, when the knee is bent. This ex- 
planation has been controverted by Vicq 
d’Azyr, and others, who have referred the 
effect to the irritability of the muscles. The 
opinion of Borelli appears, notwithstand- 
ing, to be well founded ; for not only the 
tendon of the accessory flexor passing round 
the knee, but the course of the flexor ten- 
dons over the heel, and along the metatar- 
sus, must necessarily cause the contraction 
of the toes when either of these joints is 
bent ; and if the phenomenon was not pro- 
duced on mechanic principles, it would be 
impossible for birds to exhibit it during 
sleep, which they do, or to prove the effect 
on the limb of a dead bird, than which no- 
thing is more easy. The utility of this con- 
trivance is great in all birds, but particu- 
larly so in the rapacious tribe, which, by 
this means, grasp their prey in the very act 
of pouncing on it ; and it is still more ne- 
cessary to those birds which perch or roost 
during their sleep, as they could not other- 
wise preserve their position, when all their 
voluntary powers are suspended. 
URINARY ORGANS. 
The structure of the kidney in the mam- 
malia displays two very opposite varieties, 
wliich may be called the simple and the 
conglomerated kidneys. In the former 
there is a single papilla, which is surround- 
