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 



