186 INHIBITION OF THE BEAT. [BOOK i. 



not much larger than the normal. At other times stimulation of 

 the vagus does not cause any disappearance of the beats, but the 

 intervals between the beats are much prolonged, so that the 

 rhythm is for a while very slow. It is possible that these two 

 different effects are brought about by more or less distinct 

 mechanisms. 



We said just now that after the stimulation of the vagus has 

 ceased the beats may go beyond their previous vigour and fre- 

 quency. This is sometimes remarkably the case. We might be 

 tempted to speak of it as a reaction, were it not that no necessary 

 relation obtains between the amount of slowing or weakening and 

 the amount of succeeding acceleration or augmentation. Indeed 

 the latter effect may make its appearance without any previous 

 inhibition; that is to say, under certain circumstances stimulation 

 of the vagus may produce not inhibition but either augmentation 

 of the beats, or quickening of the rhythm, or both. 



During the standstill, direct stimulation of the heart, as by 

 touching the auricle or ventricle, will produce a single beat; 

 though spontaneous pulsations are absent, the mechanism for the 

 production of a beat is capable of being put into action. 



The stimulus need not be an interrupted current; mechanical 

 and chemical stimulation of the vagus also produces inhibition, 

 though less readily. 



The stimulus may be applied at any part of the course of 

 either vagus (though it frequently happens in some animals, as in 

 the frog, that one vagus is more efficient than the other); but 

 perhaps the most marked effects are produced, when the electrodes 

 are placed on the boundary-line between the sinus venosus and the 

 auricles. 



The effects of various poisons in reference to this inhibitory 

 action are very interesting. After atropin, even in a minute dose, 

 has been injected into the blood, stimulation of the vagus even 

 with the most powerful currents produces no inhibition whatever. 

 The heart continues to beat as if nothing were happening ; atropin 

 in some way or other does away with the normal inhibitory action 

 of the vagus. 



In slight urari poisoning, the inhibitory action of the vagus 

 is still present; in the profounder stages it disappears, but even then 

 inhibition may be obtained by applying the electrodes to the sinus. 

 In order to explain this result it has been supposed that what 

 we may call the inhibitory fibres of the vagus terminate in an in- 

 hibitory mechanism (probably ganglionic in nature), seated in the 

 heart itself, and that the urari, while in large doses it may paralyse 

 the terminal fibres of the vagus, leaves this inhibitory mechanism 

 intact and capable of being thrown into activity by a stimulus ap- 

 plied directly to the sinus. After atropin has been given, inhibition 

 cannot be brought about by stimulation either of the vagus fibres 

 or of the sinus, or indeed of any part of the heart. Hence it is in- 



