96 



THE MECHANISM OF THE CIRCULATION. 



The Pulse. 



The most difficult problem connected with the hydrodynamics of the 

 circulation is that of wave motion. 



Suppose an extensile elastic tube (AB) to be divided into two parts 

 by the membrane (0) (Fig. 66). 1 If the membrane be made to move 

 towards A, a positive wave will be propagated along OA, and a negative 

 wave along OB. If, however, the membrane be imperfect, then a wave, 



_ _ - - . partly positive and 



partly negative, will 

 be propagated in 

 both directions. The 

 pulse wave is formed 

 in a similar manner. 

 Suppose to be the 

 aortic orifice, B the heart, and the membrane the aortic valve. As blood 

 is forced from B into A, it develops a positive wave along OA. The 

 fluid is driven into OA, with first, an increasing, and secondly, a 



B 



Fig. 66. 



diminishing 



velocity, and finally the force of propulsion conies to rest. 

 If we consider the tube (OA), which is now expanded, to be divided into 

 sections a, b, c, d, c, f, g, h, % (Fig. 67), 2 it is obvious that, when the 

 ventricular output ceases, pres- 



sure will be exerted by the 

 expanded elastic wall in the 

 direction shown by the arrows : 

 that is to say, the fluid particles 

 are propelled towards i, retarded 

 and brought to rest towards a. 



Thus section a returns to rest and its 



Fig. 67. 

 original diameter, 



while 



section i expands. Likewise the sections /, //, h swell, since more 

 particles of fluid are propelled through the wider sections d, c than can 

 escape in the same time through the narrow sections /, g, h. Similarly, 

 the sections a, b, c will shrink. Thus the wave of expansion travels 

 from section to section along the tube (A). Since the return of the blood 

 towards A, in the direction of the arrows, will close the aortic valve (0), 

 and drive it as a whole towards B, a negative wave will be developed 

 in OA. This wave will likewise be propagated along the tube (A). 

 On the motion towards B coming to rest, the valve (0) will again 

 move towards A, giving rise to a second positive wave, which in its 

 turn will travel down the tube (A). Thus are produced in the arterial 

 system the primary wave of expansion, and the secondary wave of 

 expansion (dicrotic wave). 



A wave in an elastic tube to a certain extent produces a translation 

 of the particles of fluid, but the wave must be regarded mainly in the 

 light of a translation of a change of form. The propulsion of each 

 particle of fluid occasions a successive disturbance of the stability of 

 contiguous particles ; thus the impact of the blood, which is thrown 

 from the heart into the aorta, travels down the arterial system. So an 

 impact travels from truck to truck, when an engine strikes the 

 end of a long railway train. If the train be in motion at the moment 



1 Nicolls, Journ. Physiol., Cambridge and London, 1890, vol. xx. p. 425. 

 3 Weber, Bcr. d. k. Sachs. Gesellsch. d. Wissenseh., Math'. -Nat. CI., Leipzig 

 S. 164-204. 



1850, Bd. i. 



