492 MUSCULAR EXERTION TO MAINTAIN EQUILIBRIUM. 
the body is placed near the front; hence the body must be 
entirely changed in its position by a violent and not sustain¬ 
able action of the muscles which connect it with the hind 
legs; and, when thus reared up, it cannot rest with firmness 
on account of the narrowness of the base. 
654. There are some Quadrupeds, however, which are able 
to raise themselves occasionally into this position ; this is the 
case, not only with the Quadrumana, but also with the Bear, 
Squirrel, and other animals whose habits require them to 
ascend and live among trees,—as well as in the Kangaroo, 
and animals constructed upon the same plan, whose peculiar 
organisation will be presently considered (§661). In standing 
upright, the muscles of the back part of the neck are kept in 
a contracted state, to retain the head in equilibrium on the 
vertebral column; and the extensor muscles of that column 
must also be kept in action, to prevent it from bending 
forwards under the weight of the head, upper extremities, 
and viscera of the trunk. The whole weight of the upper 
part of the body is thus transmitted to the sacrum, and thence 
to the other bones of the pelvis, by which it is brought to 
bear on the femur. If left to themselves, the thigh-bones 
would bend beneath the pelvis, and the trunk would fall 
forwards ; but the contraction of their extensor muscles keeps 
them firm. In the same manner, the extensor muscles of the 
knee and ankle keep these joints from yielding beneath the 
weight of the body, which is thus at last transmitted to the 
ground. The sitting posture is less fatiguing than the stand¬ 
ing position, because the weight of the body is then directly 
transmitted from the pelvis to the base of support, so that it 
is not requisite for the extensor muscles of the lower limbs 
to keep-up a sustained action. But the lying posture is that 
of the most complete rest; because the weight of every part 
of the body is at once transmitted to the surface on which it 
bears, and no muscular movement is requisite to keep it in its 
position. 
655. This difference in muscular effort, is the cause of a 
well-marked variation in the pulse, according to the position 
in which the body is at the time. From a considerable 
number of observations it has been found that the average 
pulse of an adult man is about 81 when standing, 71 when 
sitting, and 66 when lying ; so that the difference between 
