PULSE IN DIFFERENT POSTURES : ACT OF WALKING. 493 



standing and sitting is 10 beats of l-8th of the whole, 

 whilst the difference between sitting and lying is 5 beats or 

 1-1 3th of the whole. In the female, the pulse is quicker in 

 each position by from 10 to 14 beats per minute ; but the 

 differences occasioned by position are nearly the same. It 

 will be observed that the difference between standing and 

 sitting is greater than that between sitting and lying ; and 

 this closely corresponds with the relative amounts of muscular 

 exertion required in these positions respectively. At the 

 moment when the posture is changed, the pulse is considerably 

 quickened, in consequence of the muscular effort required for 

 the purpose, which acts especially on the veins, and forces the 

 blood more rapidly back to the heart ( 279); but this 

 increase in rapidity is temporary only. 



656. All that has been said of the positions of Vertebrated 

 animals applies equally well to those of the Invertebrata, 

 which like them have the body raised from the ground upon 

 extremities. This is the case in the higher Articulata, such 

 as Insects, Crustacea, Arachnida, and Myriapoda. But the 

 lower Articulata crawl, like Serpents, upon the whole length 

 of their bodies ; or, being aquatic, are buoyed-up by the 

 element they inhabit. And among the Mollusca and Eadiata, 

 there are none that have members upon which they can be 

 said to stand. 



657. The progressive movements by which the bodies of 

 Man and other animals are made to change their places, are 

 accomplished by means of the alternate contractions and 

 extensions of those limbs, which we have hitherto considered 

 only as supporting them in a rigid position. It is easy to see 

 that when a joint is straightened after being bent, the two 

 ends of the levers which form it must be separated from each 

 other, and that motion must thus be given to the parts against 

 which one or both of them bear. Now in the ordinary move- 

 ments of progression, one of these levers bears against the 

 ground, which is immovable ; and the whole motion produced 

 by straightening the joint must consequently be communicated 

 to the body. In the ordinary act of walking, one of the feet 

 is planted in front, whilst the other is extended or carried 

 backwards beneath the leg, by the action of the muscles of 

 the calf aided by those of the toes (. 648). Its length is 

 thus increased j and as it bears upon the resisting soil, this 



