STANDING POSTURE EQUILIBRIUM. 489 
shock produced by the sudden descent of the body upon the 
ground. 
Of the Attitudes of the body , and the various kinds of Locomotion. 
650. A small number of Yertebrated animals,—Serpents, 
for instance,—bear habitually on the whole length of their 
bodies, which rest entirely on the ground; and their only 
movements are effected by undulations of the spinal column. 
But the rest are supported upon their extremities ; and we 
give the name of standing to that position in which the 
animal rests supported by its limbs upon the ground or on 
any firm horizontal basis. In maintaining this position, the 
extensor muscles, by which the joints are straightened, must be 
in continual action, since the limbs would otherwise bend 
beneath the weight of the body. Now as the sense of 
fatigue, in any set of muscles, depends in great degree upon 
the length of time during which they have been in continuous 
action, the maintenance of the standing posture for a long 
period is, in most animals, more fatiguing than walking; 
since in the latter exercise the action of the flexors alternates 
with that of the extensors. 
651. But this condition is not the only one essential to 
steadiness in the standing posture; for in order that the 
body may rest firmly upon the members, it must be in equi¬ 
librium. It has been shown (Mechan. Philos. Chap, iv.) 
that equilibrium exists,—or in other words, that a body 
remains at rest in its position,—not only when it bears upon 
the whole of a broad sur¬ 
face, but also when it is 
so placed that the tenden¬ 
cies of its different parts 
to descend or gravitate 
towards the earth counter¬ 
balance each other. This 
is the case when its centre 
of gravity is supported,— 
that is, when a line drawn 
perpendicularly from that centre falls within the base. In 
order, then, that an animal may rest in equilibrium on its legs, 
it is necessary that the vertical line from its centre of 
gravity (or line of direction) should fall within the space 
