226 PHYSIOLOGICAL PHYSICS. [Chap, x XL 



The height of the pulse wave, then, reveals the 

 tension. 



The law which has been stated, that the speed of 

 propagation of the wave is proportional to the elastic 

 force of the vessel explains how, the more rigid a 

 vessel becomes (for instance, by calcification and such 

 senile changes), the faster is the transmission of the 

 pulse ; it explains, too, the length of the wave in the 

 pulse tracing to the left of the figure, and in the 

 tracing obtained, for instance, from a person suffering 

 from hypertrophied vessels, due to chronic Bright's 

 disease of the kidney. 



Again, the dependence of the extent of the wave 

 on the suddenness of the disturbance of equilibrium 

 (page 218), and on the quantity of fluid forced into the 

 vessel, by each shock, offers an explanation of the 

 abruptness that gives the "shotty" character to the 

 pulse of aortic insufficiency. 



Thus the physical conditions explain the phe- 

 nomena of the pulse. The application of what has 

 been observed as to the effects of the elasticity of 

 vessels also shows that it is to the operation of this 

 force following up the shock of the heart, that the 

 continuous flow of blood through the capillaries is 

 due. It explains why loss of this elasticity, by calci- 

 fication of the arterial walls, should be followed by 

 pulsation continued into the capillaries, and even into 

 the veins. It also explains how the work of the heart 

 is economised by the quantity of discharge being in- 

 creased through elastic tubes. 



It is now necessary to explain the methods by 

 which observations on blood pressure, arterial tension, 

 and velocity of the blood, have been made. 



Blood pressure. The figure on page 211 shows 

 how the pressure of the blood on the walls of the 

 vessels may be measured. The piezometers, described 

 on the same page are actually measurers of the force 



