THE HEART 109 



assumption that there is a considerable degree of suction 

 exerted by the ventricles during diastole.* 



If the ventricle is unable to empty itself completely at 

 each systole, any negative suction which it can exert is, 

 of course, abolished. This may be an additional reason 

 for the laboured action of a dilated heart. The part 

 played by the negative pressure in the mediastinum in 

 promoting expansion of the heart during diastole is also 

 of considerable importance, and will be considered in 

 another chapter. 



The Apex-Beat. 



The apex-beat is normally felt in the fifth intercostal 

 space at a point (in the adult) about 4J inches from the 

 mid-sternal line. The apex-beat is not felt with equal 

 ease, however, at all periods of life. Up to the age of 

 twenty or thereabouts it is always palpable in normal 

 conditions, even when the subject lies on his back ; but 

 beyond that age it can only be made out at all easily in 

 about half the individuals examined. The reason for 

 this is that the ease with which the apex-beat can be 

 determined depends upon the extent to which the heart 

 and chest wail are in contact i.e., upon the size of the 

 heart relative to that of the thoracic cavity. Now, the 

 ratio of the heart to the chest is greatest in young 

 subjects, hence the ease with which the apex-beat 

 can be made out in such cases. For this reason, too, 



* Stacey Wilson, ' The Diastolic Expansion Movement of the 

 Ventricles as a Factor in Compensation for Disease of the Mitral 

 Valve,' Brit. Med. Journ., 1900, ii. 895. 



