THE CAUSATION OF THE HEART-BEAT 



1067 



Analogous effects are obtained on leading off the spontaneously 

 beating heart in the frog or tortoise (Fig. 431). The conditions are, 

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

 has shown, is triphasic. This is probably due to the fact that the wave 

 of excitation follows the original arrangement of the muscular fibres. 

 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 arterial end. 

 In the higher vertebrates the heart at its first appearance has the 





I 





FIG. 431. Electrometer record of variation of spontaneously beating frog's 



heart. (GoTCH.) 



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. In the frog it is easy to obtain direct records of the con- 

 tractions of the different cavities, showing the regular sequence 

 through sinus, auricle, ventricle, and bulbus. The tube is, how- 

 ever, twisted on itself to form the ventricle, hence the part of the 

 ventricle in muscular continuity with the auricles is genetically 

 posterior to that part of the ventricle which immediately adjoins the 

 bulbus. Hence it comes about that in the frog's ventricle the wave 

 of electrical change which accompanies the excitatory condition is 

 triphasic. The excitatory process in the auricles is succeeded 

 immediately by activity of the base of the ventricle. The wave then 

 travels to the apex, and from here back to the base in the neighbour- 

 hood of the bulbus, so that the base, as judged by leading off to the 



