THE CAUSATION OF THE HEART-BEAT 



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cular wall, perfectly free from ganglion-cells, may maintain rhythmic 

 contractions for some hours if fed by an artificial circulation through 

 a branch of the coronary artery. We may therefore conclude that 

 in the mammalian, as in the amphibian, heart the cause of the rhythm 

 is to be sought in the properties of the muscle fibres themselves, and 

 that every part of the heart-muscle possesses the power of rhythmic 

 activity, the normal sequence of the beats being determined by the 

 greater frequency of the natural rhythm of the venous end of the 

 heart. 



As in the amphibian heart, the electrical changes may be used as 

 an index of the course and time-relations of the excitatory wave 

 in the mammalian heart. Waller first showed that the electrical 

 changes in the human heart might be recorded by leading off two 

 parts of the body (v. Fig. 432), properly placed, to an electrometer ; 



FIG. 433. Electrocardiogram of man, obtained by leading off from the two 

 hands to a string galvanometer. 



c is the carotid pulse tracing. The different parts of the curve are desig- 

 nated by the letters P, Q, K, s, T, first applied to them by Einthoven. 



and Bayliss and I, from the human heart, leading off the chest wall 

 over the apex beat and the right hand to the capillary electrometer, 

 obtained a triphasic variation similar to that recorded for the frog. At 

 that time we interpreted the curves as indicating that the excitatory 

 process at the base of the ventricle, though beginning before, out- 

 lasted the excitatory condition at the apex. More recent and better 

 curves obtained by Einthoven, both with the capillary electrometer 

 and the string galvanometer, show that the changes are somewhat 

 more complex. A human electrocardiogram, obtained by Einthoven, 

 is given in Fig. 433. In this case the right and left hands were 

 connected with the terminals of a string galvanometer. With this 

 arrangement the right hand may be regarded as leading off from the 

 base, while the left hand leads off from the apex of the heart. In 

 the curve, ' negativity ' of the base, i.e. of the auricular end of the 

 heart, is indicated by a movement upwards, and vice versa. In the 

 curve the excursion P is certainly due to the auricular contraction. 



