CHAP, iv.] THE VASCULAR MECHANISM. 153 



occupying from 40 to 46 p. c. of the whole period; and being fairly 

 constant for different rates of heart-beat. 



The observer, listening to the sounds of the heart, made a signal at 

 each event on a recording surface, the difference in time between the 

 marks being measured by means of the vibrations of a tuning fork 

 recorded on the same surface. By practice it was found possible 

 to reduce the errors of observation within very small limits. 



Now whatever be the exact causation of the first sound, 

 it is undoubtedly coincident with the systole of the ventricles, 

 though possibly the actual commencement of its becoming audible 

 may be slightly behind the actual beginning of the muscular con- 

 tractions. Similarly the occurrence of the second sound due to the 

 closure of the semilunar valves may, as we have seen, be taken to 

 mark the close of the ventricular systole. And thus the interval 

 between the beginning of the first and the occurrence of the second 

 sound has been regarded as indicating approximatively the duration 

 of the ventricular systole, i.e. the period during which the ventri- 

 cular fibres are contracting. If however we accept the view that 

 the ventricle still remains contracted for a brief period after the 

 valves are shut, then the second sound does not mark the end of 

 the systole, and the duration of the systole is rather longer than 

 the *3 sec. given above. 



The propulsion of the blood into the aorta leads to an expansion 

 of the aorta walls, known as the pulse, which we shall study more 

 fully immediately. This pulse travels, as we shall see, along 

 the arteries at a certain rate : it is later at arterial points 

 more distant from the heart than at points nearer the heart. "We 

 can calculate with approximative correctness the time it takes for 

 the expansion to travel from the aortic valves to the radial artery 

 at the wrist, for example. Now when we record, as we may do on 

 the same recording surface, the exact moment at which the first 

 sound begins, or at which the lever of the cardiograph begins to 

 rise in the ventricular systole, and also the exact moment at which 

 the expansion of the corresponding pulse at the wrist begins, and 

 measure the interval of time between them, we find that the interval 

 is greater than is required for the expansion of the pulse-wave to 

 travel from the heart to the wrist. The difference gives the measure 

 of the time during which the ventricle by its contraction is getting 

 up an adequate pressure upon its contents, and during which, as yet, 

 blood has not escaped from the ventricular cavity and begun to ex- 

 pand the aorta : the measure in fact of what we called, a little while 

 ago, the first period of the ventricular systole. This may also be 

 estimated by directly measuring the time taken up by the upstroke 

 of the cardiographic tracing, and has been said to be on an average 

 about '085 sec. These measurements however are approximative 

 only and there can be no doubt that the time varies very largely, 

 being dependent on the quantity of blood in the ventricle, on the 

 blood-pressure in the aorta and on the condition of the heart. 



