224 PHYSIOLOGICAL PHYSICS. [Chap. xxi. 



of the blood. While the rate of decrease varies, the 

 general fact remains that the pressure diminishes from 

 the aorta through the capillaries to the veins, in which 

 it is least of all. 



It has also been seen that the velocity of the flow is 

 inversely as the diameter of the tubes. Now, owing 

 to the multiplication of branches, the total diameter 

 at the capillaries is much greater than at the aorta, or 

 than at the veins opening into the heart. It is, 

 accordingly, observed that the speed of the blood 

 diminishes from the aorta to the capillaries, and then 

 increases from the capillaries to the right side of the 

 heart, though the speed at the right side does not 

 come up to the rate in the aorta, the diameter at the 

 former level being greater than at the latter. 



In considering next the part played by the 

 elasticity of the vessels, aid is also obtained from the 

 consideration of the purely physical conditions. For, 

 first of all, it is evident that the phenomenon of the 

 pulse is due to this factor, and that the characteristics 

 of the pulse are capable of affording valuable informa- 

 tion to the physiologist and physician, as to the 

 condition of the vessels and as to the character of the 

 force propelling the blood through them. From what 

 has been said (page 217) it will be understood that 

 the pulse is due to the dilatation of the artery under 

 the influence of the increased pressure transmitted 

 to the blood by the heart, and the subsequent recoil 

 of the elastic walls upon the blood within them, and 

 that this movement is not to be confounded with the 

 onward movement of the blood itself. Further, it has 

 been explained that the pressure exerted upon the 

 blood by the elastic recoil is called the tension of the 

 arterial walls. 



The characters of the wave can be made visible 

 by a graphic tracing, obtained in a way to be men- 

 tioned immediately. What it is desired to note 



