60 THE HEART 



usually attributed, like the third auricular wave, to the 

 gradual filling of the auricle, the auriculo-ventricular valve 

 being closed. 



The a wave is therefore an index of auricular, and the 

 c wave an index of ventricular contraction, while the 

 distance between them is a measure of the rate of con- 

 duction from the auricle to the ventricle. 



The Heart-Sounds 



At each beat two sounds are normally heard. The first 

 is best heard at the apex and is due to the contraction of 

 the ventricles and to the closure of the mitral valve. The 

 second sound, shorter and sharper than the first, is 

 also audible at the apex, but is heard best at the base. It 

 is caused by the sudden closure of the aortic valve. When 

 the valves are destroyed by disease, the eddies set up and 

 the flow of blood in abnormal directions cause the normal 

 sounds to be replaced by '" murmurs. 



j> 



Electrical Changes in the Heart 



The apparatus used for the detection of the current of 

 action of the heart in situ is an adaptation of the string 

 galvanometer — known as the electro-cardiograph (see p. 44). 

 Owing to the sahne content of the tissues and tissue-fluids, 

 the body conducts an electric current as though it consisted 

 merely of salt solution. When a diflerence of potential 

 occurs anywhere within the body, as in the heart, this can 

 by appropriate means be detected at the surface. Owing 

 to the obhque disposition of the heart, a potential at the 

 base tends to spread upwards and to the right, a potential 

 at the apex downwards and to the left. The subject is put 

 into circuit with the galvanometer by having his right hand 

 and left foot inserted into pots containing salt solution 

 wired up with the two ends of the string. The axis of the 

 heart is then more or less in line with the circuit, and any 

 difference of potential between base and apex of the heart 

 is recorded by the string. 



