404 ELECTRO-PHYSIOLOGY CHAP. 



current of action in the spontaneously beating frog's ventricle. As 

 may be seen, the first phase of the action current begins percep- 

 tibly earlier than the contraction ; the negativity of the former 

 (depression' of the meniscus), corresponding with the maximum 

 P.D. between base and apex, is reached long before the maxi- 

 mum of contraction, upon which a reversed current ensues as the 

 second phase, when apex becomes negative to base. In Fig. 131, 

 (t) is the time in -^ sec. The capillary electrometer is so 

 connected with the base and apex of the ventricle, that the effect 

 is downwards, when base is negative to apex. 



Cardiac response in the tortoise, and, as shown by A. D. 

 Waller and Eeid (39), in warm-blooded (mammalian) animals, is 



Kni. 131. Curve of contraction (h) and action current (c) of spontaneously beating 

 Frog's heart. (A. D. Waller.) 



also analogous with that of the frog. With artificial excitation 

 of the excised and already quiescent ventricle, the proximal 

 electrode is found to be at first negative, and immediately after 

 positive, to the distal electrode, and a diphasic variation is thus 

 produced, in consequence of the two phasic action currents, 

 similar in all respects to that of the frog's heart. Owing, 

 however, to the much greater velocity of excitation in the 

 heart of warm - blooded animals, and the abbreviated period 

 of contraction, the two phases merge into each other, as in 

 striated skeletal muscle. Fig. 132, which is a photogram of the 

 movements of the capillary electrometer with a normally beating 

 and artificially excited mammalian heart, shows plainly that each 

 phase corresponds with a simple variation, in the sense of a single 

 excitatory wave. The capillary electrometer also shows a normal 



