Page 16. 
54 “THE LAW WHICH REGULATES 
the requirements, but varies very little from them. When further 
opportunity occurs, I hope to repeat these experiments on a larger 
scale. 
It can also be shown in other ways that the arteries do not obey 
the laws of ordinary elastic tubes. They are covered by a dense, scarcely 
elastic, fibrous coat which limits their distension, and they are sur- 
rounded by organs and muscles which are pressing on them in all 
directions. So it may be said at least that they do not vary in capa- 
city as simple elastic tubes, and that the difference is towards their 
varying directly as the pressure. 
However, the indirect evidence proves that the capacity of the 
arterial system, the ventricle included, varies directly as the pressure : 
for the facts above considered as to the frequency of the pulse depend- 
ing on the resistance, and not at all on the pressure, can only bé 
explained on this assumption. 
If the direct evidence ‘as to the dapat of the wiitiela had been 
contradictory, it is true that it would have been necessary to assume 
some error in the method of conducting the pulse experiments; but as 
above shown, it is quite in the right direction and only lacks partial 
direct verification. 
So much in the verification of theories connected with Physiology 
must depend on the way in which collateral facts are explained by ; 
them that it will be advisable now to consider some of them, and these 
considerations will be divided into two sections,—I1st. The explanation 
of the known variations in pulse-rate in health; and 2ndly, The expla- 
nation of the cardiograph laws. 
1st. Variations in Pulse-rate in Health—With reference to these 
points, as on this theory change in pulse-rate can only depend on 
change in-arterial resistance, it is evident that Marey’s law will, upon 
his supposition as to the relation between blood pressure and arterial 
resistance, explain the phenomena equally well. 
The following are some of the best known :— 
The Effects of Respiration on the Pulse-rate——Physiologists, though 
not completely agreed as to the effects of respiration on the pressure 
of the blood in the arteries, all acknowledge that during inspiration 
the pulse quickens, and during expiration it gets slower, whether the 
pressure rises or falls. The theory under consideration clearly shows 
that this must be so, for during inspiration the expansion of the chest 
must reduce the pressure in the intra-thoracic aorta, and consequently 
its contained blood must fall in tension more rapidly than if the chest 
were motionless, and the more rapid tension fall causes increase in 
pulse-rate. In expiration the opposite occurs, diminution in chest 
capacity reduces the size of the aorta and consequently delays the 
time of fall of tension, and therefore slows the pulse. 
