so PATHOLOGY. 



ments in alternately opposite directions. By measuring the 

 time wliicli intervenes between the first and second vibrations, 

 the exact duration of the systole of the heart may be ascertained, 

 for the one occurs at the instant that the ventricle hardens in 

 contraction, the other at the instant that the aortic valve be- 

 comes tense in closing. 



" 4. Both movements manifest themselves with much greater 

 distinctness in some cases than in others. In this difference 

 it seems probable that the respective valves have much to do, 

 and that the systolic vibration is produced by the sudden 

 tightening of the mitral valve, just as the diastolic vibration is 

 due to the sudden tightening of the aortic valve. The precise 

 physical conditions on which the degree of vibration depends, 

 have not as yet been ascertained, but observation points to the 

 conclusion that the intensity of the systolic vibration is greatest 

 when the arterial pressure is lowest at the close of the diastolic 

 period. 



" 5. From the moment that the ventricles begin to contract, 

 the fulness of the arterial system, and consequently the arterial 

 pressure, rapidly increases. As, however, the arteries at first 

 yield readily to the tide of blood, the tension does not attain 

 its maximum until some time after the hardening of the 

 ventricles in contraction. The duration of the interval between 

 the one event and the other — that is, between the closure of the 

 mitral valve and the moment of highest pressure in the radial 

 artery — varies. It is longest when the arterial system is full ; 

 shortest when it is comparatively empty. Hence the measure- 

 ment of this interval comes to be of considerable importance. 



" G. From the moment that the artery attains its greatest dis- 

 tension, it begins to collapse ; the form of that part of the 

 sphygmographic tracing which corresponds to the period of 

 relaxation is, as has been recently pointed out by Dr. Eivers, 

 parabolic. In the normal pulse the parabolic form is not easily 

 distinguished, the line of descent being nearly rectilinear; but 

 in all those pulses in which the collapse is rapid, it is very 

 obvious, and most of all in what has been called the mono- 

 crotous — single — form, which corresponds to the thready pulse 

 of authors. 



" 7. In certain conditions of the circulation, the radial artery, 

 immediately after the distension produced by the contraction of 



