2 1 4 THE CONTRA CTION OF CARD I A C MUSCLE. 



on the tortoise, I could not find any diminution in the force of the contrac- 

 tion of the ventricle, such as is well known to occur in the frog and toad 

 (cf. Figs. 114 and 115 and Figs. 108 and 109), but, on the contrary, have 

 always found that the ventricular contractions are unaltered in strength 

 if the rate of beat is unaltered (Fig. 114), or increased in strength 

 according to the amount of slowing of the rate of beat caused by the 

 stimulation of the nerve (Fig. 115); and MacWilliam 1 says that during 

 the standstill due to vagus stimulation there is no diminution of excit- 

 ability in its ventricle, as certainly occurs in the case of the frog. The 

 same indifference of the ventricle to vagus stimulation was seen in the 

 case of the eel by MacWilliam, of the crocodile and snake by me, of the 

 terrapin and toad-fish by Wesley Mills ; 2 in other words, in all the cases 

 of Teleostei, Chelonia, and Iieptilia which have been observed, the vagus 

 does not in any way affect the ventricle. On the other hand, the vagus 

 does affect the ventricle in the same way as the auricle — though to a 

 less degree — in the frog and toad, in the newt according to MacWilliam, 

 and in Menobranchus according to Wesley Mills, 3 all instances which 

 are included in Amphibia. 



Further, in absolutely strict accordance with this absence of effect 

 upon stimulation of the nerve in the neck, is the absence of inhibitory 

 effect upon direct stimulation of the muscle. 



Thus, as already mentioned, direct stimulation of the ventricle of the 

 frog produces a local diastole which may last for some time, and is 

 removable by atropine — produces, in fact, on the frog's ventricle an 

 inhibitory effect which exactly corresponds with what I have described 

 as the effect of direct stimulation of the suspended strip of the auricle 

 of the tortoise. With, however, the suspended strip of the ventricle of 

 the tortoise matters are very different. Here, just as in the case of the 

 auricle, a series of blockings to the passage of the contraction are first 

 got rid of, so that finally each of the single induction shocks causes a 

 contraction which passes freely from one end of the strip to the other, but 

 at no time does the passage of a weak interrupted current diminish the 

 force of these contractions ; even if the strip is beating spontaneously 

 there is no evidence of any inhibition when the interrupted current is 

 sent through. 



A precisely similar negative result upon direct stimulation of the 

 ventricle has been observed by me in the crocodile, and by MacWilliam 

 in the eel. 



Another somewhat unexpected coincidence is brought out by the 

 comparison of ventricular muscle whose contractions are diminished by 

 vagus stimulation, and ventricular muscle whose contractions are not so 

 diminished : namely, that the staircase phenomenon obtains only in the 

 former case and not in the latter. 



This phenomenon of the staircase (I'reppe) was discovered, as has 

 been already mentioned, by Bowditch 4 in the apex preparation of the 

 frog's ventricle, and consists in this, that in the quiescent apex, through 

 which a series of efficient single induction stimuli are sent, the 

 first contraction is not the strongest, but the contractions increase in 

 staircase fashion, until a certain maximum is reached at which the con- 



1 Journ. Physiol., Cambridge and London, 1885, vol. vi. p. 224. 



2 Ibid., vol. vi. p. 246 ; and vol. vii. p. 81. 

 :! Ibid., 18S6, vol. vii. p. 81. 



- 1 Op. cit. 



