220 THE CONTRACTION OF CARDIAC MUSCLE. 



The most marked after-effect of the stimulation of the inhibitory 

 nerve is its effect upon the conductivity of the tissue. When a block 

 has been caused by a partial section of the auricle in the tortoise, so 

 that only every second contraction passes the block, it is very common 

 to see, as an after-effect of vagus stimulation, that every contraction 

 passes, and with frequent vagus stimulation I have found that more and 

 more tissue must be cut away, in order to keep the contractions blocked. 

 Similarly, in the frog's heart, the clamp in the auriculo-ventricular 

 groove requires to be screwed up tighter and tighter after a series of 

 stimulations of the intracranial vagus, in order to cause a partial block. 

 We see, in fact, just the reverse of what occurs in the dying heart, and 

 also the reverse of what is seen after stimulation of augmentor fibres ; 

 for in the latter case I have noticed that after a series of stimulations 

 of the augmentor nerve in the frog or toad, a partial block at the 

 auriculo-ventricular junction, so that the ventricle responds to every 

 second auricular contraction, is very apt to be produced, although with 

 the clamp in the same position it responded to every contraction before the 

 commencement of the series of stimulations. So, too, with respect to the 

 force of the contractions, a series of stimulations of the augmentor nerve 

 distinctly exhaust the muscular tissue of auricle and ventricle, so that 

 it contracts more weakly than it would have done in the same time 

 if no nerve had been stimulated. On the other hand, a series of 

 stimulations of the intracranial vagus in the frog or toad keeps the 

 heart up to a condition of vigorous contraction, which affords a very 

 striking contrast to the condition of exhaustion of the heart which has 

 been acted on by the augmentor nerve. So, too, after repeated stimula- 

 tions of the augmentor nerve, not only does the heart beat weakly, but 

 also these weak contractions cannot be augmented on stimulation of 

 the nerve to any very great extent ; under these circumstances I have 

 frequently seen a decided improvement in the augmentor action, in 

 consequence of the interpolation of two or three intracranial vagus stimu- 

 lations. In fact, the evidence as to contraction-force and conductivity is 

 distinctly to the effect that in each case the after-effect is in the opposite 

 direction to the primary effect. 



The after-effect upon rhythm is not so clear. My own experience 

 is that, if I have been stimulating the augmentor nerve alone for a long 

 time at intervals, the net result has been a quick rate of beat throughout 

 the whole tune ; and if I have been stimulating in the same way the 

 intracranial vagus alone, then, throughout, the heart has been beating 

 slowly. I do not mean to assert that there is never any evidence of an 

 increase of rate after each vagus stimulation, but simply that the 

 exhaustion of the rhythmical tissue of the sinus, which is presumed to 

 follow after repeated stimulations of the augmentor nerve, does not 

 manifest itself in a marked slowing of the beat, any more than the 

 presumed beneficial effect of a series of vagus stimulations manifests 

 itself by a marked quickening of the beat. I do not feel clear how far 

 it is possible to argue with respect to rate in the same way as with 

 respect to force of contractions ; an exhausted muscle must necessarily 

 contract more weakly, but does it necessarily follow that an exhausted 

 rhythmical muscle beats at a slower rate of rhythm ? 



The conclusion to which I came in 1881 l was that the vagus nerve 

 was the anabolic nerve of the heart, and that inhibition or relaxa- 



1 Phil. Trans., London, vol. clxxiii. p. 1029. 



