THE CIRCULATION OF THE BLOOD. 



357 



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. All conditions which 

 modify the rate of the heart will modify at the same time the rate of the pulse. 



The Sphygmograph. The sphygmograph is 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 following each 



FIG. 169. VON FREY'S SPHYGMOGRAPH. G. S. Metal framework. P. Button attached to spring. 

 F. Vertical rod. U. Clock-work which turns the recording cylinder. VI. Time marker. 



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

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

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

 of v. Frey is shown in Fig. 169. This consists first of a metal framework 

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

 lever, recording surface, etc. The essential part is the spring carrying a but- 

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

 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 OR SPHYGMOGRAM OF THE RADIAL 

 the radial. This artery lies quite superficially, ARTERY - 

 is covered only by connective tissue and skin and is supported by the flat 

 surface of the radial bone, conditions most favorable to technical investiga- 

 tion. An average tracing taken from the radial artery is shown in Fig. 170. 

 This, however, is not a tracing of the pulse-wave, but rather a record of 

 the changes in pressure, their succession and time relations, which follow 

 each beat of the heart. 



The sphygmogram or pulse-curve may be divided into two portions: 



FIG. 170. THE PULSE-CURVE 



