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46 THE LAW WHICH REGULATES 
2. “The frequency of the pulse varies inversely as the arterial 
tension.” 
As reasons for the accuracy of this law are given— 
Ist. The analogy of other intermittent muscular movements, as 
the following :—A man can walk a certain distance quicker, the less 
he is loaded. Or this—The hand can be moved alternately backwards 
and forwards more quickly in air than in the more resisting fluid, water. 
2nd. The pressure can be made to change by variations in the 
amount of blood in circulation, and by modifications in the degree of 
arterial or capillary resistance, both of which vary the pulse-rate in 
the manner required by the theory. 
To prove the effects of different amounts of blood in circulation, 
the experiments of Hales are quoted, in which he found that loss of 
blood increased the frequency of the pulse. 
To prove the effects of varied arterial or capillary resistance, many 
satisfactory and original results are referred to; among them, the 
effect of compressing the abdominal aorta, or the femorals, which 
retards the pulse; the effects of cold baths, according to Drs. Bence 
Jones and Dickinson, when the pulse was greatly reduced in fre- 
quency ; the quickened pulse following successive additions of warm 
clothing over the body is also proved. : 
From these latter results it is clear that Marey assumes that by 
varying the capillary resistance the blood pressure is also varied at 
the same time, but this assumption is not necessarily true in a circu- 
lation that is maintained by a pulsating motor organ whose rate is 
variable, as can be easily shown by an analogy from electricity, which 
is a useful one in many ways to students of the circulation, and is 
quite worth being worked out by each. It is this :—Suppose a battery 
connected, through a break-and-make key, to a long uniform insulated 
line or telegraph cable, insulated at the other end, and connected with 
a static galvanometer. 
First connect the two parts by the key and thereby charge the line, 
and then break connection; upon this the charge will fall in tension 
slowly, and this fall may be observed on the galvanometer; when the 
tension has fallen one half, reconnect and break again. It is evident 
that if this process be repeated, a definite current is maintained 
between the cable and the surrounding bodies to which it leaks. If 
the line be now halved in length, whereby the resistance is doubled, 
and again insulated at the free end, it is evident that by again break- 
ing and making contact as before, when the tension is halved, the 
maximum tension will not be changed. So with the circulation, if 
the resistance in the arterial peripheral vessel is varied and the length 
of the pulsation depends on the time of fall in tension only, the 
pressure does not vary, if the vascular capacity is constant. 
