492 MUSCULAR EXERTION TO MAINTAIN EQUILIBRIUM. 
the body is placed near the front; hence the body must b 
entirely changed in its position by a violent and not sustair 
able action of the muscles which connect it with the hin 
legs ; and, when thus reared up, it cannot rest with firmnes 
on account of the narrowness of the base. 
654. There are some Quadrupeds, however, which are abl 
to raise themselves occasionally into this position ; this is th 
case, not only with the Quadrumana, but also with the Bea 
Squirrel, and other animals whose habits require them t 
ascend and live among trees,—as well as in the Kangaroc 
and animals constructed upon the same plan, whose peculiz 
organisation will be presently considered (§ 661). In standin 
upright, the muscles of the back part of the neck are kept i 
a contracted state, to retain the head in equilibrium on th 
vertebral column ; and the extensor muscles of that colum 
must also be kept in action, to prevent it from bendin 
forwards under the weight of the head, upper extremitie 
and viscera of the trunk. The whole weight of the upps 
part of the body is thus transmitted to the sacrum, and thenc 
to the other bones of the pelvis, by which it is brought 1 
bear on the femur. If*left to themselves, the thigh-bone 
would bend beneath the pelvis, and the trunk would fa 
forwards ; but the contraction of their extensor muscles keey 
them firm. In the same manner, the extensor muscles of th 
knee and ankle keep these joints from yielding beneath th 
weight of the body, which is thus at last transmitted to th 
ground, The sitting posture is less fatiguing than the stance 
ing position, because the weight of the body is then direct! 
transmitted from the pelvis to the base of support, so that 
is not requisite for the extensor muscles of the lower liml 
to keep-up a sustained action. But the lying posture is th: 
of the most complete rest ; because the weight of every pa 
of the body is at once transmitted to the surface on which 
bears, and no muscular movement is requisite to keep it in i 
position, 
655. This difference in muscular effort, is the cause of 
well-marked variation in the pulse, according to the positic 
in which the body is at the time. From a considerab 
number of observations it has been found that the averag 
pulse of an adult man is about 81 when standing, 71 whe 
sitting, and 66 when lying ; so that the difference betwee 
