EQUILIBRIUM OF ANIMALS :—BASE OF SUPPORT. 491 
the tarsus, c the hone of the metatarsus termed the canon (in 
which the trace of a division into two pieces can he seen), 
and p, pi , pt, the three phalanges of the toes, of which the 
last is enveloped in the hoof, which is nothing else than a 
large nail inclosing the whole extremity of the toe. In the 
Horse this consolidation is carried still further than in the 
Ruminants, for it has only one toe in each foot (fig. 233); hut 
we see the rudiment of an additional hone in the metatarsus 
6, which is commonly termed the splint hone. 
653. But when an animal is supported upon two feet only, 
whatever may he their degree of separation from each other, 
the hase of support cannot have sufficient extent, unless the 
extremities touch the ground hy a considerable surface. This 
is the case with the foot of Man, and still more with that of 
many Birds which habitually stand 
upon one leg (fig. 234). In order 
that an animal may hold itself in 
equilibrium upon a single limb, it 
is necessary that the foot should 
he placed vertically beneath the 
centre of gravity of the body; 
and that its muscles should he so 
arranged as to permit it to keep 
this limb inflexible and immov¬ 
able. Man can accomplish this, 
for the centre of gravity of his 
body is at about the middle of the 
pelvis ; and to place this vertically 
over one foot, it is sufficient for 
him to bend himself a little from 
the side which is not supported. 
But the greater number of Qua¬ 
drupeds are destitute of the power 
of doing this; and a large part 
of them cannot even raise them¬ 
selves on their hind legs, on 
account of the direction of these members relatively to the 
trunk; or if they can do so for an instant, they cannot 
maintain themselves in this position. The reason of this is 
very simple. The base of support, on account of the small¬ 
ness of the feet, is very narrow, and the centre of gravity of 
Fig. 234. 
