THE CAUSATION OF THE HEART BEAT 



991 



to the base line. Relaxation, like contraction, starts first at the base 

 and proceeds thence to the apex. There is thus a small period during 

 which the apex is still contracted while the base is relaxed and the apex 

 is therefore negative to the base. This terminal negativity of the apex 

 is shown on the capillary electrometer by the excursion of the column 

 of mercury away from the point of the capillary (cp. Fig. 87, p. 231). 



Analogous effects are obtained on leading off the spontaneously 

 beating heart in the frog or tortoise (Fig. 438). The conditions are 

 however rather more complex, and the most usual variation, as Gotch 

 has shown, is triphasic. In its most primitive form the vertebrate heart 

 is composed of a simple tube, in which a contraction starts at the venous 

 end and is propagated in a wave-like manner along the tube to the 



FIG. 438. Electrometer record of variation of spontaneously beating tortoise 



heart. (GoTCH.) 



arterial end. In the higher vertebrates the heart at its first appear- 

 ance has the same tubular form, but the simple tube very rapidly 

 becomes modified, partly by twisting on itself, partly by the outgrowth 

 of the dorsal or the ventral wall of the tube to form the cavities of the 

 auricle and ventricle. Gotch suggests that the excitatory process follows 

 the course of the original tube, and that the typical form of the curve 

 is due to the base becoming excited twice, first at the part in con- 

 tinuity with the auricle, and secondly when the wave sweeps up to 

 the bulbus aorta3. But it is possible that in the cold-blooded as in 

 the mammalian heart, there may be a special conducting tissue which 

 leads the excitatory process to many different parts of the ventricle 

 almost simultaneously. 



There is no doubt that the ventricular systole is comparable with a 

 simple muscular twitch and cannot be regarded as the summation of several 

 contractions. Since the excitatory process extends in the form of a wave 



