8o A MANUAL OF PHYSIOLOGY 



rate of the heart is variable. If we take about 70 beats a minute 

 as the average normal rate in a man, the ventricular systole will 

 occupy about 0-3 second ; the diastole,* including the ventricular 

 relaxation, about 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 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. 82) ; 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 in a bloodless heart the valves are not stretched. 

 Part of the sound must accordingly 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 inaudible. But while there is good reason to believe 

 that the vibration of the suddenly-contracted ventricles is the 

 fundamental factor, the shock sets up vibrations also in the 

 blood, the chest-wall, and perhaps the resonant tissue of the lungs. 

 Further, as we shall see later on (p. 660), the sound caused by a 

 contracting muscle readily calls forth sympathetic resonance in 



* The term ' diastole ' is variously used, as meaning the pause, the 

 pause plus the period during which relaxation is occurring, or the period 

 of relaxation alone. We shall employ it in the second sense. 



