The Bloodvessels. 63 



further the tube is removed from the central pump ; this 

 mechanical law applies to elastic as well as rigid tubes ; 

 but as applied to the bloodvessels, we find that instead of 

 there being a gradual reduction in pressure, the latter 

 decreases irregularly owing to the force being used up in 

 overcoming the peripheral resistance. In arteries, for 

 example, the pressure diminishes slowly; in capillaries, 

 owing to the increased resistance, the reduction is very 

 rapid, whilst in the veins it decreases slowly. 



The velocity of flow is inversely as the sectional area of 

 the tubes ; the total sectional area of the capillaries is 

 greater (about 500 times) than that of the aorta, therefore 

 the velocity is reduced ; from the capillaries to the heart 

 the area becomes smaller and the velocity increases. The 

 narrower the vascular bed the more rapid the flow, the 

 wider the bed the slower the flow. 



If fluid be forced into an elastic tube, and the flow main- 

 tained under constant pressure, the fluid will behave as if 

 being forced through a rigid tube ; but if the pressure be 

 rendered an inconstant one, the elastic walls distend at the 

 increase of pressure and recoil when the pressure is re- 

 duced ; this distension and recoil occurs throughout the 

 whole length of the tube, and produces a wave which 

 we term in living tubes the pulse. We must be careful to 

 avoid the error of considering that this wave represents the 

 flow of blood in the vessel, it does not ; it is simply the 

 outcome of an impression at one end of the tube, which is 

 travelling very much faster along the walls of the tube 

 than the blood which is within. 



The rate at which the wave travels depends on the elas- 

 ticity of the walls : the more elastic the tube the slower the 

 wave and the greater its height ; the more rigid the tube 

 the faster the wave and the less its height. 



By an application of these physical principles to the living 

 body, we are able to account for many of the physiological 

 problems of the circulation in bloodvessels ; the intermittent 

 action of the heart, the elastic walls of the arteries, the 

 peripheral resistance, the increase in the width of the 



