242 COMPARATIVE PHYSIOLOGY. 



syringe, imitating' the heart, and against a resistance made by 

 drawing out a glass tube to a fine point and inserting it into 

 the terminal end of the rubber tube, an intermittent pressure 

 like that occurring in the artery may be observed ; and further 

 that it does not occur at precisely the same moment at the two 

 points tested. 



Information more exact, though possibly open to error, may 

 be obtained by the use of more elaborate apparatus and the 

 graphic method. 



By measurement it has been ascertained that in man the 

 pulse-wave travels at the rate of from five to ten metres per sec- 

 ond, being of course very variable in velocity. It would seem 

 that the more rigid the arteries the more rapid the rate, for in 

 children with their more elastic arteries the speed is slower ; 

 and the same principle is supposed to explain the higher veloci- 

 ty noticed in the arteries of the lower extremities. But with 

 such a speed as even five metres a second it is evident that with 

 a systole of moderate duration (say "3 second) the most distant 

 arteriole will have been reached by the pulse-wave before that 

 systole is completed. 



It is known that the blood-current at its swiftest has no 

 such speed as this, never perhaps exceeding in man half a metre 

 per second, so that the pulse and the blood-current must be two 

 totally distinct things. 



When the left ventricle throws its blood into vessels already 

 full to distention, there must be considerable concussion in con- 

 sequence of the rapid and forcible nature of the cardiac systole, 

 and this gives rise to a wave in the blood which, as it passes 

 along its surface, causes each part of every artery in succession 

 to respond by an elevation above the general level, and it is this 

 which the finger feels when laid upon an artery. 



That there is considerable distention of the arterial system 

 with each pulse may be realized in various ways, as by watch- 

 ing, and feeling an artery laid bare in its course, or in very thin 

 or very old people, and by noticing the jerking of one leg 

 crossed over the other, by which method in fact the pulse-rate 

 may be ascertained. And that not only the whole body but 

 the entire room in which a person sits is thrown into vibra- 

 tion by the heart's beat, may be learned by the use of a tele- 

 scope to observe objects in the room, which may thus be seen to 

 be in motion. 



Features of an Arterial Pulse-Tracii^.— In order to judge of 



