THE CIRCULATION. 



289 



sixths are occupied by its reaction. At more remote points the 

 time of expansion becomes longer and that of collapse shorter ; unttf 



FIG. 67. 



CURVES OF PULSATION IN AN ELASTIC TUBE. 1. Near the distending force. fc. At a distance 

 from it. 3. Still farther removed. 



finally, at a certain distance, the amount of expansion is reduced one- 

 half, and the two periods are equalized in duration. 



Registration of Pulse by the Sphygmograph. The frequency and 

 character of the arterial pulse may be permanently recorded by the 

 use of an instrument similar in principle to the cardiograph, but adapted 

 for application to an artery. This instrument, of which there are vari- 

 ous modifications, is the Sphygmograph. It consists essentially of a 

 small ivory or metallic plate, gently pressed upon the artery by 

 means of a fine spring, so as to rise and fall with the expansion 

 and collapse of the arterial tube. The plate communicates its motion, 

 through a vertical metallic rod, to a registering lever above. The 

 oscillating extremity of the lever, when the instrument is in opera- 

 tion, thus indicates the movements of the artery, and marks upon 

 a strip of paper the frequency and form of its pulsations. 



The advantage of such an instrument is, first, that the length of the 

 lever magnifies to the eye the extent of the arterial oscillations, and 

 thus enables us to perceive movements too delicate to be distinguished 

 by the touch ; and, secondly, that, each part of a pulsation being per- 

 manently registered, the most momentary changes may be afterward 

 studied at leisure and compared with each other. 



By this means it has been shown, that, while there is a general 

 resemblance in the form of pulsation of different arteries, nearly every 

 vessel to which the instrument can be applied presents peculiarities 



FIG. 68. 



TRACE OF THE RADIAL PULSE, taken by the Sphygmograph. 



dependent on its size, position, and distance from th,e heart. In the 

 radial artery at the wrist, each pulsation consists of a sudden expan- 

 sion of the vessel, indicated by a rapid upward movement of the lever, 

 making, in the trace, a straight, nearly vertical line. This is fol- 



T 



