A CTION OF THE INHIBITOR Y NER FES. 2 1 5 



tractions remain steady for some time. MacWilliam x has been unable 

 to confirm this observation of Bowditch when the ventricle of the eel or 

 tortoise is used instead of that of the frog. In these cases he says there 

 is no sign of any ascending staircase, the first contraction is as large as any 

 of the others. I have seen the same thing in the strip of the tortoise 

 ventricle ; here it is true that the single induction shocks, when first sent 

 into the strip, cause very small contractions, and gradually and more or 

 less steadily the contractions increase in size, until they attain their 

 maximum ; this phenomenon, however, is clearly due to the gradual 

 removal of the blocking points in the strip. When the strip has recovered, 

 so that every contraction passes freely from end to end, then I have found 

 that the first contraction after a pause, whether long or short, is as large 

 as any of the subsequent contractions. It is very suggestive in this con- 

 nection, that Bowditch 2 found that atropine removed this staircase effect 

 in the frog's ventricle. The atropinised frog's ventricle, in fact, behaves 

 in this respect, as in so many others, similarly to the non-atropinised 

 ventricle of the tortoise, and this suggests that the phenomenon of the 

 Trep'pe is clue to the direct stimulation of inhibitory fibres in the frog's 

 ventricle. 



The investigation of the primary action of the vagus on the heart of 

 cold-blooded animals brings out clearly two facts of great importance — 



1. The inhibitory action is a direct one of nerve fibre upon muscle 

 fibre, and affects all the properties of the muscle fibre in the same way — 

 rhythmicity, excitability, conductivity, contraction-force, and tonicity. 



2. The cold-blooded vertebrates are divisible into two sets, namely, those 

 of the amphibian type, in which the ventricular muscle is affected in the 

 same way as the auricular, but to a less degree, and those of the same 

 type as Teleostei, Chelonia, and Eeptilia, in which the inhibitory action 

 of the vagus nerve appears to be confined to the auricles ; at all events, 

 no action is apparent on the ventricles. 



This extraordinary and unexpected difference in the behaviour of the vagus 

 nerve in these two groups leads at once to the idea that we are dealing here 

 with a deep-seated morphological difference, and that the study of the physio- 

 logy of the cardiac nerves may give very important help in the classification of 

 the vertebrata ; thus, if the birds have arisen from reptiles and the mammals 

 from amphibia, we might expect to find that the vagus nerve does not affect the 

 ventricle in the bird's heart, but does act upon the mammalian ventricle in the 

 same way as upon that of the frog or toad. As far as the latter question is 

 concerned, the experimental researches of Mac William 3 point to the conclusion 

 that the vagus does diminish the force of the ventricular as well as the 

 auricular contractions in the various mammalia investigated (cat, dog, rabbit, 

 rat, hedgehog, guinea-pig), and indeed that the nerve acts upon rhythm, 

 excitability, contraction-force, etc., of the different parts of the heart, precisely 

 in the same way as in the case of the amphibian ; so that, as far as the heart of 

 the mammal is concerned, the physiologist would agree that it is closely related 

 tu the amphibian. With respect to the heart of the bird, I have been unable 

 to find any evidence ; 4 I cannot find that anyone has ever observed the effect of 



1 Joum. Physiol., Cambridge and London, 1885, vol. vi. p. 212. " Op. clt. 



3 " Inhibition in the Mammalian Heart," Joum. Physiol., Cambridge and London, 18S8, 

 vol. ix. p. 345. 



4 Knoll, who has recently published a paper on the action of the vagus nerve in warm- 

 blooded animals (Arch. f. d. yes. Physiol., Bonn, Bd. xlvii. S. 595), descril es some experi- 

 ments on the hearts of [liaeons as well as those on mammals. In this paper he points out 

 that stimulation of the vagus nerve does diminish the strength of the contractions of the 



