STANDING POSTURE :—EQUILIBRIUM. 489 
shock produced by the sudden descent of the body upon the 
rs ound 
Of the Attitudes of the body, and the various kinds of Locomotion. 
650. A small number of Vertebrated 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 
xtensor muscles, by which the joints are straightened, must be 
n continual action, since the limbs would otherwise bend 
neath the weight of the body. Now as the sense of 
5 ~y ue, in any set of muscles, depends in great degree upon 
he 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 
ith that of the extensors. 
_ 651. But this condition is not the only one essential to 
gadiness 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 (Mecuan. Puitos. Chap. 1v.) 
oot exists,—or in other words, that a body 
s at rest in its position,—not only when it bears upon 
the F-whole of a broad sur- 
f but also when it is 
ie iplaved that the tenden- 
ties of its different parts 
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 FE. 
P erpendicularly from that centre falls within the base. In 
der, then, that an animal may rest in equilibrium on its legs, 
is necessary that the vertical line from its centre of 
ity (or line of direction) should fall within the space 
