62 CIRCULATION. 



on Bourdon's principle is nearly free from " proper motion," 

 and consequently enables us to measure the fine variations 

 of arterial pressure in the course of each pulse interval. 



The pulse as felt by the finger indicates the moment of 

 greatest distension, i.e. that of greatest pressure in the 

 artery. The distinctness with which it is felt is propor- 

 tional to the shock communicated to the arteries by the 

 heart. Pulses are classified according to frequency, hard- 

 ness, duration and dicrotism. All of these characters may 

 be appreciated by the finger, but are studied more accu- 

 rately by the sphygmograph. 



Three events may be distinguished in every pulse, viz., 

 the beginning of the expansion, the collapse, and the 

 beginning of the second beat. Of these the first occurs 

 in the normal radial pulse about 0*15 second after the 

 beginning of the effective part of the systole of the left 

 ventricle. The second is synchronous with the end of 

 the systole, and hence immediately precedes the closure 

 of the sigmoid valves. The third is synchronous with, or 

 immediately after, the closure of the valves. 



The graphical characters of the arterial pulse, e.g. the 

 radial, are determined by (a) the character of the diastolic 

 notch which, in the tracing, separates the first from the 

 second ascent, and (b) the relative height of the second 

 ascent. The notch is produced by the sudden cessation of 

 the flow of blood from the ventricle into the aorta. It 

 may be generally stated that the shorter the duration of 

 the systole and the less the vascular resistance, the deeper 

 is the notch. 



The second beat is determined entirely by events which occur in the arteries 

 and capillaries. When any part of an artery is suddenly distended, all parts 

 of the arterial tree beyond pulsate after it ; but each pulsates at a different 

 moment, according to its distance from the par* primarily distended, and as 

 each attains its .maximum of distension, it sends back a return wave of expan- 

 sion. The moment at which the strongest return waves, and the greatest 

 number of them, arrive at the point of observation, is that at which the second 

 beat occurs. 



