THE HEART BEAT. 545 



ventricles. In this tissue the conduction is very rapid, about 3000 

 to 5000 mms. per second. By means of this system of so-called 

 Purkinje fibers the excitation is distributed nearly uniformly to the 

 musculature of the two ventricles. It was the older view that the 

 excitation and contraction reached first some definite region and 

 thence followed the anatomical course of the fibers, but according 

 to Lewis the excitation is conveyed by the conducting system almost 

 simultaneously to the whole of the interior surfaces of the ven- 

 tricles, and at each point radiates out through the muscular mass 

 toward the external surface. , The result is that the whole mass of 

 the musculature contracts practically at once, although careful 

 galvanometric examination of the external surface by the method 

 referred to in the next paragraph reveals that the excitation reaches 

 the surface first where the musculature is thinnest or is connected 

 most directly with the A-V bundle. 



The Electrical Variation. The contraction of the heart mus- 

 cle, like that of skeletal muscle, is accompanied or preceded by 

 an electrical change. That is, where the muscle substance is in 

 contraction its electrical potential is different from that of the 

 resting muscle. The advancing wave of contraction causes a 

 corresponding electrical change or, to be more accurate, the ad- 

 vancing wave of excitation, which precedes the actual contraction, 

 is accompanied by an electrical change. If two points of the heart 

 are connected with an electrometer an electrical current will be 

 shown, since the electrical change will affect the electrodes at 

 different times. This electrical variation of the contracting heart 

 muscle may be shown easily by means of the rheoscopic muscle- 

 nerve preparation (see p. 104). If the heart is exposed and the 

 nerve of the preparation is laid over its surface each ventricular 

 systole is accompanied by a kick of the muscle, since the nerve by 

 connecting separated points acts as a conducting wire for the 

 current generated, and is stimulated, therefore, at each systole. 

 Since the muscle-nerve preparation gives only a simple contrac- 

 tion for each ventricular systole, we may assume that this latter 

 contraction is itself simple, that is, due to a single stimulus. 

 The electrical variation may be obtained also by means of the 

 capillary electrometer or the string-galvanometer (p. 99), and 

 since the movement of the mercury or of the string in these in- 

 struments may be photographed, the results can be studied in 

 detail. Owing to the sensitiveness of the instrument, the beat 

 of the human heart may be registered in this way (Waller) when 

 the right hand, giving the potential changes of the base of the 

 heart, is connected with one electrode, and the left hand (apex 

 of heart) is connected with the other. The electrocardiograms 

 thus obtained photographically show that, in the ventricle at 

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