7o THE MECHANISM OF THE CIRCULATION. 



great, and then with an ever-decreasing velocity, until the velocity reaches 

 zero at the moment when the pressure in D and D' becomes the same. It is 

 therefore clear that the flow of fluid is produced by the difference of pressure 

 in A and V. If this pressure difference be greater, the velocity of flow is 

 greater ; if it be smaller, the velocity is smaller. 



So soon as the pressure difference between the artery and vein is estab- 

 lished, the fluid will move through the sponge into the vein, not in jets, 

 but with a constant flow. If in the model, H be working with a constant 

 rhythm and energy, and a constant flow be established, it is obvious that 

 any change in the output per second of H, or in the peripheral resistance, 

 will upset the constancy of the flow, until a new condition of equilibrium 

 is once more established. 



In such a continuous system as Weber's model, a hydrostatic mean pressure 

 is obtained when the heart is at rest. This is the average fluid pressure, 

 as already stated, which pertains to the system, when the capacity of the model 

 is constant, and the fluid is in a state of stasis. 



The hydrokinetic mean pressure is the average fluid pressure throughout 

 the system when the fluid is in motion. These means are obtained by 

 measuring the pressure at each unit of length along the tube and dividing 

 by the number of units. Weber maintained that the hydrostatic and hydro- 

 kinetic mean pressures are equivalent ; that is to say, the pump cannot 

 increase the mean pressure of the system. By so much as it increases the 

 pressure on the arterial side it will decrease it on the venous side. This 

 deduction of Weber's is not admissible, for the arterial tension can be 

 raised, while the tension in the vein cannot fall below zero, for, as already 

 stated, the flaccid wall of the vein is unable to support any part of the atmo- 

 spheric pressure. 



It has also been held that contraction of arterioles, by the influence of vaso- 

 motor nerves, not only increases the peripheral resistance, but also diminishes 

 the total capacity of the system, so that we have the same effect produced as if 

 more fluid were poured into the system. Thus, it has been said, an increase in 

 peripheral resistance must always connote a rise in the hydrostatic mean 

 pressure of the system brought about by the diminution of its capacity. 

 According to this supposition, a rise of arterial tension, occasioned by vaso- 

 motor activity must be explained by the coincidence of two factors — (a) 

 increased peripheral resistance ; (b) diminished capacity of blood vessels. 1 

 Now, if the model be not filled to distension, then any diminution of its 

 capacity will not raise the fluid hydrostatic pressure above zero, until the 

 reduction of capacity has reached the point when the walls of the system are 

 thrown into tension. In other words, if 1000 c.c. be introduced within an 

 elastic bag, capable of holding without distension 2000 c.c, the capacity of the 

 bag can be reduced from 2000 c.c. to 1000 c.c. without any alteration of the 

 tension of the wall. On turning to the evidences of experiment, we find that 

 the vascular system is not filled to distension. The proofs for this statement 

 are the following : — 



1. Owing to the elastic resilience of the arteries, and the influence of 

 gravity, the blood, after the arrest of the heart, passes into the venous side, and 

 to the most dependent parts of the body. Thus the large arteries are found 

 after death to be empty of blood. 



2. It is not possible to distend the entire vascular system of a recently 



killed animal even by injecting into an artery a volume of fluid equal to that 



of the blood. Even after the injection of 500 c.c. of water into a dog, the 



tension in the arteries falls with the utmost rapidity. The fluid flows until it 



has collected in the most dependent parts of the body. By alternately placing 



the animal in the horizontal and vertical feet-down postures, the fluid can be 



1 Bayliss and Starling, Journ. Physiol., Cambridge and London, 1894, vol. xvi. p. 165 ; 

 cf. Mall, Arch./. Physiol., Leipzig, 1892, S. 409. 



