88 THE CIRCULATION OF THE BLOOD AND LYMPH 



0-5 second. The systole of the auricle is one-third as long as that of 

 the ventricle. 



This rhythmical beat of the heart is the ground phenomenon of 

 the circulation. It reveals itself by certain tokens sounds, surface- 

 movements or pulsations, alterations of the pressure and velocity of 

 the blood, changes of volume in parts all periodic phenomena, 

 continually recurring with the same period as the heart -beat, and all 

 fundamentally connected together. And if we hold fast the idea 

 that when we take a pulse-tracing, or a blood-pressure curve, or a 

 plethysmographic record, we are really investigating the same fact 

 from different sides, we shall be able, by following the cardiac rhythm 

 and its consequences as far as we can trace them, to hang upon a 

 single thread many of the most important of the physical phenom- 

 ena of the circulation. 



The Sounds of the Heart. When the ear is applied to the chest, or 

 to a stethoscope placed over the cardiac region, two sounds are 

 heard with every beat of the heart. They follow each other closely, 

 and are succeeded by a period of silence. The dull booming ' first 

 sound ' is heard loudest in a region which we shall afterwards have 

 to speak of as that of the ' cardiac impulse ' (p. 90) ; the short, sharp, 

 ' second sound ' over the junction of the second right costal cartilage 

 with the sternum. 



The heart-sounds can be registered by placing over the chest a 

 microphone receiver connected with a string galvanometer. The 

 magnified sounds are translated into electrical changes which cause 

 movements of the fibre of the galvanometer, and the movements are 

 photographed on a travelling plate (Einthpven). The record is called 

 a cardiophonogram. When this is studied, a third sound can be 

 detected, and it is probable that it is present in all persons, although it 

 is as a rule inaudible to auscultation. It occurs early in diastole very 

 shortly after the second sound. In those persons in whom it is audible 

 it is most distinct over the region of cardiac impulse. It is described 

 as softer and of lower pitch than the second sound (Thayer). 



There has been much discussion as to the cause of the first sound. 

 That a sound corresponding with it in time is heard in an excised 

 bloodless heart when it contracts is certain ; and therefore the first 

 sound cannot be exclusively due, as some have asserted, to vibra- 

 tions of the auriculo-ventricular valves when they are suddenly 

 rendered tense by the contraction of the ventricles, for in a bloodless 

 heart the valves are not stretched. Part of the sound must accord- 

 ingly be associated with the muscular contraction as such. 



Again, the fact that the first sound is heard during the whole, or 

 nearly the whole, of the ventricular systole is against the idea that 

 it is exclusively due to the vibrations of membranes like the valves, 

 which would speedily be damped by the blood and rendered in- 

 audible. But while there is good reason to believe that the vibra- 

 tion of the suddenly-contracted ventricles is the fundamental factor, 

 the shock sets up vibrations also in the blood, the chest-wall, and 



