236 CIRCULATION OF THE BLOOD. 



every normal tracing begins, expresses not the more or less 

 gradually increasing arterial distension, but the antecedent 

 transmission of a vibration. 



42. Postponement of the Pulse. There is a sensible dif- 

 ference in time between the beat of the carotid arteiyand that 

 of the radial. Any one can satisfy himself of the fact by feel- 

 ing his own carotid with the left thumb and forefinger, while 

 he feels the left radial with the other hand. The reason why 

 time is lost in the transmission of the expansion from the centre 

 to the periphery, is that the arteries are elastic. Let us sup- 

 pose a tube, A, B, c, to represent 



the arterial system A the proximal end, c the distal. At the 

 instant that blood bursts suddenly out of the contracting heart 

 into A, it yields to the pressure against its internal surface and 

 expands. In this expansion great part of the sensible motion 

 of the blood momentary disappears, and consequently, so long 

 as the expansion lasts, produces comparatively very little effect 

 in distending B ; but immediately that A becomes tense, the 

 lost, or rather converted, motion again becomes sensible, and 

 adds itself to the motion which the contracting heart is still 

 communicating. And, inasmuch as B deals with the accumu- 

 lated effect which it receives from A in exactly the same way as 

 A dealt with that which it received from the heart, c is as far 

 behind B in attaining its maximum of distension as B was 

 behind A. This being the case, it is easy to see that the loss 

 of time between A and c, or between aorta and radial, depends 

 on the yieldingness (extensibility) of the tube by which the two 

 points are connected. If the tube is absolutely rigid, there is 

 no postponement; if, though elastic, it is tense at the moment 

 that it receives the discharge, there is scarcely any ; whereas 

 that condition of the tube is most favorable to postponement, 

 in which it is longest in attaining its maximum of distension, 

 or in which the time taken by any part of it to expand to the 

 uttermost is longest. 



The preceding explanation relates exclusively to so much of 

 the pulsation as is due to the communication of pressure. As 

 regards the antecedent vibration-effect, we have also time occu- 

 pied in transmission, but the rate of propagation is so rapid 

 that in the case of an artery, or of an elastic tube of similar 

 length, it is inappreciable. This fact enables us to explain, 

 how it is that in some persons the pulse seems to be much 

 more postponed than in others. The reason of this is, not that 

 there is more time lost in the former case than in the latter, 

 for even if this were so the difference would be certainly too 

 inconsiderable to be judged of by the finger, but that in some 



