ee ee 
in verlébraled Animals, 301 
enabled to advarice with various degrees of rapidity, and the ad- 
vantages they derive from the position of their scales, and their 
connexion with the ribs. The tortoise, the frog, the lizard, and 
various other animals of the same. class, were noticed as forming 
links in the chain of gradation leading to the more perfect con- 
formation, with reference to rapid progressive motion, which ob- 
tains in warm-blooded quadrupeds. In these, the body being 
raised higher on the limbs enjoys a greater range of motion, 
and requires a less frequent repetition of steps in traversing an 
equal space, The different proportions in which the weight of 
the body is sustained by the fore and hind extremities, the uum- 
ber of levers of which they are composed, their relative oblijnity, 
the mode in which the muscular force is disposed, aud the com- 
binations of action which result, were severally explained. A 
particular acccunt was given of the paces of quadrupeds, such 
as walking, the trot, the gallop, the amble; the bounding of 
deer, the springing of beasts of prey, the undulating pace of the 
camelopard, and the peculiarities in the progression of animals 
formed for leaping, as the hare, the jerboa, and the kangaroo. 
A gradation was pointed out in the structure of the hind foot, 
which, in monkeys, makes the nearest approach to. the human 
structure. Dr. Roget observed, that the great features of di- 
stinction between the mechanism of the human frame and that 
of quadrupeds, are derived from the former being adapted to the 
maintenance of the erect posture. 1n man, the attitude of stand- 
ing is a position of less security than it is in quadrupeds, and is | 
maintained by a succession of actions by which the centre of 
gravity is perpetually shifted from side to side; its tendency to 
fall in any one direction being immediately counteracted by small 
and insensible movements in the contrary direction. On this 
principle he also explained the security of the rope-dancer.. The 
human arm, being exempted from the office of supporting any 
part of the weight of the trunk, may be employed exclusively as 
an organ of apprehension; and the circumstances in the struc- 
ture of the several joints of the limb, and more especially of the 
hand, which render it so admirable a mechanical instrument, 
were fully pointed out. The passage of the tendons by which 
the last joints of the fingers are bent, through a perforation in 
those which are employed to bend the middle joints, was parti- 
cularly selected as an example of artificial contrivance. 
The progressive motion of birds was the next object of inquiry. 
In order that an animal may, possess the faculty of flying, two 
principal conditions, it was observed, are required; first, great 
strength of muscle to produce sufficient velocity of motion in the 
wing; and, secondly, great extent of surface in that part of the 
wiug destined to act upon the air, None of the nammalia, ex- 
cept 
