APPLICATIONS TO PHYSIOLOGY 285 



Chronophctography. — We have already explained this 

 phenomenon as due to the sudden hardening of the 

 ventricles, which, although relaxed and tensionless 

 during the stage of passive filling, become more or less 

 spherical and hard when active contraction begins, 

 they then actually exercise pressure on the blood 

 which has previously distended them. This theory 

 alone accounts for all the phenomena which can 

 be observed; it explains why the pulsation of the 

 heart is perceptible at all points of the surface of 

 the ventricle, it renders the fact intelligible, which 

 appears at first paradoxical, namely, that the heart 

 presses against the thoracic parietes, not when it 

 expands, but when it diminishes in volume. It is 

 not by an alteration in volume, but by an alteration in 

 hardness, that the heart repels everything that has 

 a tendency to compress it. The maximum degree of 

 hardness corresponds, as we before mentioned, to the 

 systole of the ventricles, namely, at the moment when 

 their powerful muscular fibres compress the blood and 

 project it into the arterial system. 



Such is the mechanism which causes the sudden 

 pulse which feels like a shock to the finger, and which 

 we call the pulsation, to suggest an analogy between 

 it and the pulse at the wrist ; that it consists of a 

 sudden rise in tension in the organ can be proved 

 by touching its hardened surface with the finger. 



This theory becomes more intelligible if the ventricles 

 of a large animal are held in the hand ; if they are 

 compressed by the fingers, there is a distinct sense of 

 resistance at the moment when the surface of the heart 

 is made tense, which intimates that the systolic 

 contraction of the muscular fibres is in process. 



We endeavoured to render this phenomenon visible 

 to the eye by the following experiment, in which we 

 made use of an arrangement of this kind. The 

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