GENERAL PRINCIPLES OF THE CIRCULATION. 



69 



thus two negative waves would appear ; one of these would travel through A, 

 the other through V. As on diastole the valves (d) shut to while the valves (d') 

 open, the fluid from V alone can stream into H, and a negative wave appears 

 in V only. Thus the model is so constructed that, during the periodic 

 systole and diastole, only waves of positive pressure appear in A, and waves of 

 negative pressure in V. Both waves aid one another, and cause the fluid to 

 move in one and the same direction. 



If the system were formed of wide tubes, free from constrictions, each 

 positive wave would travel with so great a velocity, that the whole system 

 would reach the same pressure, and before the next systole occurred a 

 condition of equilibrium would be established. On the other hand, on account 

 of the resistance in the sponge, which represents the arterioles and capillaries, 

 it is otherwise. The internal friction or viscosity of the fluid particles, as they 

 pass through the interstices of the sponge, prevent the fluid from pressing 

 through with anything like the velocity of transmission of the positive wave. 

 Thus the positive wave becomes reflected at the sponge, and runs to and fro in 



Fig. 46. — Distribution of pressure and velocity during diastole and 

 systole of the heart. — Fredericq. 



A, until it is finally damped down and lost, so that no pulse reaches V. 

 Should, however, the sponge be loose in structure, and possess wide pores, then 

 the pulse might pass through into V. 



If the periodic systole of H be made to occur with a sufficient frequency, 

 the fluid is piled up in A, for at each systole a fresh quantity of fluid is 

 driven into A, while, in the same time, a less quantity escapes through the 

 sponge into V. 



With each systole the pressure mounts in A. In V, the pressure, on the 

 other hand, may sink, but it cannot sink below zero, in spite of the continued 

 abstraction of fluid, owing to the fact that the flaccid wall of V cannot support 

 any part of the atmospheric pressure. Suppose the pressure in the arterial 

 manometer (D) has risen to 0, and in the venous manometer has sunk to 

 p, and at this point imagine the action of // to cease, the pressure will 

 fall in D and rise in D', at first with greater and then with decreasing 

 velocity, until the pressure in the two manometers is once more the same. In 

 other words, the action of H has upset the balance of tension, and this balance 

 is regained by the flow of fluid in the direction of the arrows, at first with 



