470 CIRCULATION OF BLOOD AND LYMPH. 



lies somewhere in the aorta, but the pressure that has been trans- 

 mitted along the system and has caused it to expand has made 

 room for the blood forced out of the aorta by the new blood. 

 With the cessation of the heart beat and the closure of the semilunar 

 valves, the sharp recoil of the distended aorta drives forward the 

 column of blood, and as the aorta sinks back to its normal diastolic 

 diameter the more distal portions of the arterial system are at first 

 distended to a certain point and then return to their diastolic size 

 as the excess of blood streams through the capillaries into the veins. 

 At the time that the aorta has reached its diastolic size the walls 

 of the most distant arterioles have passed their maximum extension 

 and are beginning to collapse. The distension caused by the pulse, 

 therefore, spreads through the arterial system in the form of a wave. 

 At any given point the distension of the walls increases to a maxi- 

 mum and then declines, and when this change in size is recorded 

 in the large arteries, by methods described below, it is found that 

 the expansion of the artery is much more sudden than the subse- 

 quent collapse. This difference is understood when we remember 

 that the heart throws its load of blood into the arteries with sud- 

 denness and force, causing a sharp rise of pressure, while the collapse 

 of the arteries is due to their own elasticity. The disappearance 

 of the pulse before reaching the capillary area is readily compre- 

 hended when one remembers that the arterial tree constantly in- 

 creases in size as one passes out from the aortic trunk. Many facts, 

 such as those of pressure and velocity already described, indicate 

 that the increase in capacity of the arterial system is somewhat 

 gradual until the region of the smallest arterioles and capillaries is 

 reached and that at this point there is a sudden widening out or 

 increase in capacity of the whole system, although the individual 

 vessels diminish in diameter. It is in this region that the pulse 

 becomes imperceptible.* 



Velocity of the Pulse Wave. From the above considerations 

 it is evident that in a system such as is presented by our blood- 

 vessels the velocity of the pulse wave must vary with the rigidity 

 of the tubes. If perfectly rigid the pressure would be transmitted 

 practically instantaneously; if the walls were very extensible the 

 prepagation would be relatively slow. For our blood-vessels as 

 they exist at any given moment the velocity of the pulse wave may 

 be estimated by a simple method : Two arteries may be selected at 

 different distances from the heart and the pulse wave as it passes by 

 a given point in each artery may be recorded by some convenient 



* For a satisfactory discussion of the pulse and for literature consult von 

 Frey, "Die Untersuchung des Pulses." Berlin, 1892. For a description of 

 the variations in disease consult Mackenzie, " The Study of the Pulse, etc." 

 New York, 1902. 



