

CIECULATION 245 



higher than the arterial pressure (Latent Period}. Immediately 

 the semilunar valves are forced open and a rush of blood occurs 

 from the ventricles (Period of Overflow). This usually lasts less 

 than - 2 second. If the ventricles are acting powerfully, and 

 if the pressure in the arteries does not offer a great resistance to 

 the entrance of blood, the ventricles rapidly empty themselves 

 into the arteries, and the in tra- ventricular pressure varies as 

 shown in fig. 124, b, p. 267. If the heart, however, is not acting 

 forcibly, or if the arterial pressure offers a great resistance to the 

 entrance of blood, then the outflow is slow and more continued 

 and in this case the trace of the in tra- ventricular pressure 

 is like fig. 124, a, with a well-marked Period of JResidual 

 Contraction. It is not 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 com- 

 pressed by the contracting muscle of the heart, and it is only 

 when the compression is removed in diastole that blood rushes 

 into them. This 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 in systole. As the 

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

 hence the sudden slight downstroke (fig. 115, 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). 

 The pressure of the apex disappears as the ventricles relax (e\ 



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. 116). (Practical Physi- 

 ology.} 



By placing a finger on the cardiac impulse while listening 



