THE ARTERIAL PULSE 



213 



FIG. 85. Schema illustrating E. H. Weber's 

 theory of the pulse. 



will suppose, has driven the water from the rigid tube (fc) into the distensible 

 tube (ai), with a velocity at first increasing and then diminishing, and has thus 

 dilated the tube, while the water contained in the different segments has been 

 given a velocity indicated by the number of dotted arrows in each. If then the 

 ring-shaped sections of the wall inclosing the segments exert a pressure upon 

 the contained liquid, the amount of which is represented by the solid-line arrows, 

 it is evident that the particles of 

 water contained in the segments 

 e, f, 9, h, will be accelerated in the 

 direction i (since they were already 

 moving in this direction). On the 

 other hand the particles contained in 

 the segments d, c, b, a will be re- 

 tarded in their movement, since the 

 pressure indicated by the solid ar- 

 rows is exerted in the direction of Tc. 

 For this reason the liquid in a comes 



to rest within the next few moments, and the distended wall of this segment 

 returns to its original diameter. During the same time, the water in segment i, 

 which until now had not been moved, is pushed forward and its wall is distended. 

 Thus the wave is propagated from one segment to another in the direction of 

 the dotted arrows (E. H. Weber). The water presses upon the wall of the tube, 

 the wall in turn presses upon the water, and the wave spreads with a velocity (F), 

 which is inversely proportional to the square root of the specific gravity (A) 

 of the liquid, and of the internal diameter of the tube (d) ; directly proportional 

 to the square root of the wall's thickness (a) and of its elastic coefficient (e) 



(Moens). The law is expressed by the following formula V = k \/ - in which 



V &d 



A: is a constant and g is the acceleration of gravitation. 



The wave is changed in form more or less in its propagation through the 

 tube by the resistance due to friction. Its height is less and its length greater 

 than if there were no friction. 



The moment an elastic tube, already filled with an incompressible liquid, 

 receives an extra quantity, a wave of increased pressure is started, and is propa- 

 gated along the tube. If the flow be maintained for a time, the pressure keeps 

 a certain level for each point along the tube, the value of which is determined 

 by the same laws that apply to the flow of a liquid in rigid tubes (cf. page 198). 



If one end of an elastic tube filled and distended with water be suddenly 

 relaxed by removal of a quantity of water, a fall in pressure is propagated in 

 the form of a negative wave to the other end of the tube. Likewise if a regular 

 current flowing through an open elastic tube be suddenly checked, a negative 

 wave is set up which travels in the direction of the current. 



Besides, if the tube be not so long that waves thus set up entirely disap- 

 pear, as the result of friction, new ones will arise by reflection from the end of 

 the tube, which will materially affect the wave movements. If the end of the 

 tube where the reflection occurs be closed, the wave will be reflected with the 

 same sign, a positive wave as a new positive wave, a negative wave as a new 

 negative one. If the end of the tube be open, the wave will be reflected with 

 its sign reversed, a negative as a positive and a positive as a negative. The 

 same wave may by repeated reflection run the length of the tube several times. 

 If the end of the tube be only partially closed, every primary positive wave will 

 be transformed into a reflected one which is partly positive and partly negative. 

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