THE CIRCULATION OF THE BLOOD 



367 



Frequency of the Pulse. As the pulse or the arterial expansion and 

 recoil is the direct result of the heart's action, its frequency must, under 

 physiologic conditions, coincide with that of the heart. (See page 290.) All 

 conditions which modify the rate of the heart will modify at the same time 

 the rate of the pulse. 



For the purpose of accurately studying and analyzing the pulse-wave and 

 its characteristic features, it is necessary to obtain graphic records of the 

 alternate expansion and recoil of the artery under normal and abnormal 

 conditions. This is accomplished by means of 



The Sphygmograph. The sphygmograph is, therefore, an apparatus 

 designed to take up, reproduce, and record the alternate expansion and recoil 

 of an artery caused by the temporary increase and decrease of pressure fol- 

 lowing each heart-beat. The tracing or record obtained with it is termed the 

 pulse-curve or the sphygmogram. Different forms of this apparatus have been 



\1L 



FIG 172 -VON FREY'S SPHYGMOGRAPH. GS. Metal framework. P. Button attached to spring. 

 [P. Vertical rod. U. Clock-work which turns the recording cylinder. VI. T 



devised by Marey, Dudgeon, v. Frey, and many others. The instrument 



of v. Frey is shown in Fig. 172. This consists, first, of a metal frameworl 



GS by which the apparatus is fastened to the arm and support given 



lever, recording surface, etc. The essential part is the spring carryin 



ton P, which is placed over the artery, usually the radial, before it crosses tl 



wrist-joint. A vertical rod F transmits the 



movement of the spring to the recording lever; 



the movements of the latter are recorded on a 



small cylinder inclined slightly so that the 



upstroke may be vertical. A small electro- 



magnet serves to record the time relations of the 



changes in the blood-pressure. The artery . . 



usually selected for obtaining a sphygmogram is O^HYGMOGBAM 



the radial. This artery lies quite superficially, ' 



FlG . i 73 ._THE PULSE-CURVE 

 o, THE 



