620 REVIEW-BRIDGEWATER TREATISES, NO. 5. 
have seen, is not so well calculated for the simple support of the 
trunk, is, on the other hand, better adapted to give it those 
impulses which are to effect its progressive movements. The na¬ 
ture of these movements, and the order in which they succeed each 
other, are different according to the peculiar mode of progression 
which the animal practises, the degree of speed which it is de¬ 
sirous of exerting, and the particular end it has in view. The 
paces of a quadruped, usually distinguished, are the walk , the 
trot , the gallop , the amble , and the bound . 
“ In slow walking , only one foot is raised from the ground at 
the same moment, so that three points of support always exist 
for sustaining the weight of the body. If the centre of gravity 
be situated, as it generally is, nearly over the middle of the 
quadrangular base formed by the feet while they rest upon the 
ground, the first effort to advance which the animal makes 
propels the centre of gravity forwards. This it accomplishes by 
pressing one of its hind legs against the ground, which, by being 
thus fixed by the resistance it there meets with, becomes the 
fulcrum of the first movement. The extensor muscles of the 
limb are now exerted in giving the body an impulse forwards. 
As soon as this impulse has been given, the muscles which had 
been in action are relaxed, and the leg is raised from the ground, 
brought forwards, and laid down close to the fore foot of the 
same side. This fore foot is next raised and advanced ; and then 
the same succession of actions take place with the hind and the 
fore foot of the other side. 
“ An attentive examination of the conditions of these successive 
positions will shew that, amidst all the changes which take place 
in the points of support, the stability of the body is constantly 
preserved. It is an elementary proposition in mechanics, that 
all that is necessary for ensuring the support of the body on any 
given base, is, that the vertical line drawn from the centre of 
gravity shall fall within that base. When the animal is standing, 
the feet form a quadrilateral base; and the centre of gravity is 
in a vertical line passing either through the centre of the base, 
or as, for the reasons already mentioned more frequently hap¬ 
pens, through a point a little in front of the exact centre. At 
the time when the hind foot which began the action is raised 
from the ground, the centre of gravity, having been by this 
action impelled forwards, still remains above the base formed by 
the other three feet, and which is now reduced to a triangle. 
The hind foot being set down, while the corresponding fore foot 
is raised, a new triangular base is formed by the same hind foot, 
together with the two of the other side, which have not yet been 
raised. The centre of gravity is still situated above this new 
