THE CIRCULATION 231 



so much the absolute force of the cardiac contraction or the 

 absolute intra-arterial pressure which governs this, as the 

 relationship of the one to the other. The heart may not be 

 acting very forcibly, but still if the pressure in the arteries is 

 low its action may be 'relatively strong. 



The Coronary Arteries, unlike all the other arteries, are 

 filled during ventricular diastole. During systole they are 

 compressed by the contracting muscle of the heart, and it 

 is only when that compression is removed in diastole that 

 blood rushes into them and helps to dilate the ventricles. 



The interpretation of the various details of the Cardiogram 

 is now rendered more easy. The ventricles, still full of 

 blood, are suddenly pressed against the chest wall. As the 

 blood escapes into the arteries they press with less force, and 

 hence the sudden slight downstroke (Fig. 113, b to c). But so 

 long as the ventricles are contracted the apex is kept tilted 

 forward, and hence the horizontal plateau is maintained 

 (c to d) and disappears as the ventricles relax (e). 



8. Sounds of the Heart. On listening in the region of the 

 heart, a pair of sounds may be heard with each cardiac cycle, 

 followed by a somewhat prolonged silence. These are known 

 respectively as the First and Second Sounds of the Heart 

 (Fig. 114). (Practical Physiology, Chap. XL) 



By placing a finger on the cardiac impulse while listening 

 to these sounds it is easy to determine that the first sound 

 occurs synchronously with the cardiac impulse i.e. syn- 

 chronously with the ventricular contraction. 



It develops suddenly, and more slowly dies away. In 

 character it is dull and rumbling, and may be imitated by 

 pronouncing the syllable lub. In pitch it is lower than the 

 second sound. 



The second sound is heard at the moment of ventricular 

 diastole. Its exact time in the cardiac cycle has been deter- 

 mined by recording it on the cardiac tracing by means of a 

 microphone. It develops suddenly and dies away suddenly. 

 It is a clearer, sharper, and higher pitched sound than the 

 first. It may be imitated by pronouncing the syllable dupp. 



According to the part of the chest upon which the ear is 



