258 CIRCULATION. 



effects than when the pressure of blood in them is very great. 

 Any circumstance which facilitates the flow of blood from 

 the arteries into the capillaries, will, of course, relieve the 

 tension of the arterial system, lessen the obstacle to the force 

 of the heart, and increase the amplitude of the pulsation ; 

 and vice versa. In support of this view, Marey has found 

 that cold applied to the surface of the body, contracting, as 

 it does, the smallest arteries, increases the arterial tension 

 and diminishes the amplitude of the pulsation ; while a mod- 

 erate elevation of temperature produces an opposite effect. 



In nearly all the traces given by Marey, the descent of 

 the lever indicates more or less oscillation of the mass of blood. 

 The physical properties of the larger arteries render this 

 inevitable. As they yield to the distending influence of the 

 heart, reaction occurs after this force is taken off, and, if the 

 distention be very great, gives a second impulse to the blood. 

 This is quite marked, unless the tension of the arterial system 

 be so great as to offer too much resistence. One of the most 

 favorable conditions for the manifestation of dicrotism is 

 diminished tension, which is always found coexisting with a 

 very marked exhibition of this phenomenon. 



The delicate instrument employed by Marey enabled him 

 to accurately determine and register these various phenomena, 

 by observations on arteries of the human subject and animals ; 

 and by means of an ingeniously constructed " schema" rep- 

 resenting the arterial system by elastic tubes, and the left ven- 

 tricle by an elastic bag, provided with valves, acting as a syr- 

 inge, he satisfactorily established the conditions of tension, 

 etc., necessary to their production. In this schema, the regis- 

 tering apparatus, simpler in construction than the sphygmo- 

 graph, could be applied to the tubes with more accuracy 

 and ease. 



He demonstrated, by experiments with this system of 

 tubes, that the amplitude of the pulsations, the force of the 

 central organ being the same, is greatest when the tubes are 

 moderately distended, or the tension of fluid is low, and vice 



