THE ARTERIAL PULSE 215 



be regulated by means of another screw. The whole apparatus is fastened by 

 means of a band to the lower end of the forearm (Fig. 87). Many modifications 

 of this instrument are in use. 



The method of air transmission is often used also, especially when it is 

 desired to register the pulse curves of two or more arteries, or the pulse curve 

 and cardiogram, at the same time. A receiving tambour of about the same con- 



FIG. 87. Marey's sphygmograph as used. 



struction as that described for the apex beat is fastened over the artery and is 

 connected in the usual way with the recording tambour (cf. Fig. 10, page 12). 



The sphygmograph has often ~been tested and it has been found to give the 

 pulse movements with surprising accuracy. It has been shown, for example, 

 that it reproduces very exactly the waves, already known, by other means, to 

 occur in an elastic tube (Mach) ; that the pulse curve has exactly the same 

 appearance when the pulsations are recorded without magnification and where 

 the inertia of the lever is thus reduced to a minimum (Marey) ; and that pulse 

 curves having exactly the same form as those recorded by the sphygmograph, are 

 obtained if a very small mirror is fastened to the skin over an artery, so that 

 the light may be reflected on a wall. 



But we must not suppose that every sphygmograph records pulse waves so 

 perfectly. It often happens on the contrary that the instrument distorts the 

 curve considerably. It is, therefore, necessary in every exact study of the pulse 

 by the graphic method to assure oneself of the efficiency of the instrument and 

 of the maximal speed permissible for the lever. 



The velocity of the pulse wave is measured by taking at the same time 

 pulse tracings from two arteries separated by some distance from each other. 

 The velocity of propagation found in this way varies with different individuals, 

 and with the same individual under different circumstances. In a healthy 

 man it amounts to 7-10 m. per second. The velocity is greater, the greater 

 the coefficient of elasticity. It increases therefore with a rise of blood pressure, 

 for, as we have seen at page 201, the coefficient of elasticity of the arterial 

 wall increases, at least within certain limits, as the internal pressure increases. 



The length of the pulse wave A. is obtained from the formula n\= h; 



or X = JL 9 where h is the velocity of transmission and n the rhythm. Since 



with each ventricular systole the blood is driven into the aorta for 0.2 of a 



second, the rhythm number is 5. With a velocity of 8 m. the length would 



g 

 be A.= = 1.6 m. In a grown man the distance from the heart to the 



