PULSE IN DIFFERENT POSTURES : - ACT OF WALKING. 493 
standing and sitting is 10 beats or l-8th of the whole, 
whilst the difference between sitting and lying is 5 beats or 
1-13th 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 Badiata, 
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; and as it bears upon the resisting soil, this 
