THE VASCULAR MECHANISM. 193 



an impulse will be felt corresponding with each heart beat. It is 

 most distinctly felt, and is often also visible, in the fifth intercostal 

 space, about an inch below and slightly internal to the nipple. The 

 impulse is due to a combination of two causes. In the first place, the 

 left ventricle, which lies in contact with the chest wall, and is soft and 

 flabby during diastole, becomes hard and tense with the onset of 

 systole. The sudden hardening of the ventricle gives a push to 

 the soft tissues of the chest wall with which it is in contact, thereby 

 giving rise to the cardiac impulse. In the second place, the curved 

 aortic arch tends to straighten out when the tension within it is raised 

 by the entrance of blood from the heart. The same phenomenon may 



FIG. 64. A cardiograph. (From Messrs Baird & Tatlock.) 



be readily observed in any curved elastic tube filled with fluid, into 

 which more fluid is suddenly forced. The posterior end of the aortic 

 arch rests against the vertebral column and ribs, and cannot alter its 

 position ; the anterior end, to which the heart is attached, is, therefore, 

 pushed more firmly against the chest wall. 



Although this impulse is often spoken of as the apex beat, the 

 area of the left ventricle which is in contact with the chest wall is 

 some distance above the actual apex of the heart. 



A graphic record of the cardiac impulse (cardiogram) is obtained 

 by means of an instrument known as a cardiograph. One form of 

 cardiograph (fig. 64) consists of a tambour, the membrane of which 

 is provided with an ivory button which can be placed on the chest at 

 the position of the cardiac impulse : the tambour is connected with a 

 second tambour provided with a recording lever. s Another method of 



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