THE CIRCULATION OF THE BLOOD AND LYMPH 77 



with the same period as the heart-beat, and all funda- 

 mentally 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 phenomena 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. 79) ; the short, sharp, ' second 

 sound' over the junction of the second right costal cartilage 

 with the sternum. 



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 vibrations of the auriculo- 

 ventricular valves when they are suddenly rendered tense 

 by the contraction of the ventricles, for, of course, in a 

 bloodless heart the valves are not stretched. Part of the 

 sound must accordingly be associated with the muscular 

 contraction, as such. 



As we shall see (p. 576), the sound caused by a contracting muscle 

 is probably, in part at least, a resonance tone of the ear ; and the 

 vibrations in the muscle which call forth this resonance tone are not 

 necessarily produced by a series of regularly repeated contractions 

 and relaxations. This lessens the difficulty of understanding how a 

 simple non-tetanic contraction like that of the heart should give rise 

 to a * muscular ' sound of definite pitch. 



Further, 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 dumped 

 by the blood and rendered inaudible. But there is undoubtedly 

 a valvular as well as a muscular factor involved ; and, indeed, 

 there is reason to believe that the valvular note is the essen- 



