viii BLOOD-STEEAM : MOVEMENT IN VESSELS 241 



detail of the propagation of the pulse -wave in elastic tubes. 

 Besides the classical researches of E. H. Weber (1850), we have 

 the observations of Bonders (1859), Marey (1875), and Moens 

 (1880). In rubber tubes the rate of propagation of the wave 

 varies, according to different observations, from 10 to 18 m. per 

 second. 



In an elastic tube, thrown into tension by a fluid, and closed 

 at both ends, it is possible to evoke negative as well as positive 

 'waves, generated not by the sudden rise, but by the sudden fall of 

 pressure. It is only necessary to let a small quantity of fluid 

 escape suddenly from one end, or, after compressing the tube at one 

 point, suddenly to release the compression, in order to produce the 

 transmission of a wave, represented not by a dilatation but by an 

 undulatory depression. The velocity of propagation of the negative 

 wave is practically the same as that of the positive wave, and 

 essentially obeys the same law. 



When the rubber tube in which the positive or negative wave 

 is produced is not so unduly long that the wave has died out at 

 the extreme end, the first wave propagated through the tube gives 

 rise to a second reflex wave, which traverses the entire tube in the 

 opposite direction and interferes with the primary wave, since 

 it has the same velocity of transmission. 



In elastic tubes which branch like the arterial system, the 

 waves generated in the principal vessel' extend to all the com- 

 municating branches, and at the points at which the vessels branch, 

 where there is a sudden rise of resistance, there is invariably a 

 formation of reflex waves. These reflex waves are, however, lost 

 when they reach the main vessel, which in consequence of its 

 capacity and the great elasticity of its walls acts as a kind of 

 extinguisher to the small waves reflected from the secondary 

 vessels. The aorta must act in this way in regard to the reflex 

 waves from the bifurcation points of all the other arteries 

 (Marey). 



Having thus discussed the general laws of pressure, circulatory 

 velocity, and pulse-wave in the arteries, we must next consider 

 the most important data established by the study of these three 

 complex phenomena. 



IV. The idea of measuring the blood pressure in the arteries 

 originated with Stephen Hales (1733). He connected the artery 

 of a horse with a long glass tube in order to see the height to 

 which the blood would rise. In this way he ascertained that the 

 arterial pressure was equal to a column of blood of 8 to 9 feet. He 

 further noted that the height of the column of blood in the 

 tube oscillated with the cardiac systole. Poiseuille (1828) replaced 

 Hales' piezometer by a U-shaped mercury manometer, which was 

 a great advance in practical method. To this Ludwig (1847) 

 added a float provided with a pen, which records every variation 



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