294 AUGMENTOB FIBRES. [BOOK i. 



prolonged inhibition may be produced by prolonged stimulation ; 

 indeed by rythmical stimulation of the vagus the heart may be 

 kept perfectly quiescent for a very long time and yet beat vigor- 

 ously upon the cessation of the stimulus. In other words, the 

 mechanism of inhibition, that is the fibres of the vagus and 

 the part or substance of the heart upon which these act to- 

 produce inhibition, whatever that part or substance may be, are 

 not readily exhausted. Further the inhibition when it ceases is, 

 frequently at all events, followed by a period of reaction, during 

 which the heart for a while beats more vigorously and rapidly 

 than before. Indeed the total effect of stimulating the vagus- 

 fibres is not to exhaust the heart but rather to strengthen it ; and 

 by repeated inhibitions carefully administered, a feebly beating 

 heart may be nursed into vigorous activity. 



The other set, those joining the vagus from the sympathetic, 

 are 'augmentor' or 'accelerating' fibres; the latter name is the 

 more common but the former is more accurate since the effect of 

 stimulating these fibres is to increase not only the rapidity but 

 the force of the beat ; not only is the diastole shortened but the 

 systole is strengthened, sometimes the one result and sometimes 

 the other being the more prominent. In contrast with the case of 

 the vagus fibres, a somewhat strong stimulation is required to 

 produce an effect ; the time required for the maximum effect to 

 be produced is also remarkably long. Moreover, at all events in 

 the case of a heart in which the circulation is not maintained and 

 which is therefore cut off from its normal nutritive supply, the 

 augment or fibres are far less easily exhausted than are the inhibi- 

 tory fibres. Hence when, in such a heart, both sets of fibres are 

 stimulated together, as when the vagus trunk in the neck is 

 stimulated, the first effects produced are those of inhibition, but 

 these on continued stimulation may become mixed with those of 

 augmentation and finally the latter alone remain. Lastly the 

 contrast is completed by the fact that the augmentation resulting 

 from the stimulation of the sympathetic is followed by a period of 

 reaction in which the beats are feebler ; in other words augmen- 

 tation is followed by exhaustion ; and indeed by repeated stimula- 

 tion of these sympathetic fibres a fairly vigorous bloodless heart 

 may be reduced to a very feeble condition. 



By watching the effects of stimulating the sympathetic nerve 

 at various points of its course we may trace these augmentor 

 fibres from their junction with the vagus down the short sympa- 

 thetic of the neck through the first splanchnic or sympathetic 

 ganglion connected with the first spinal nerve, G 1 , Fig. 55, 

 through one or both the loops of the annulus of Vieussens, 

 An. V, through the second ganglion, connected with the second 

 spinal nerve, G H , to the third ganglion connected with the third 

 spinal nerve, G 111 , and thence through the ramus communicans 

 or visceral branch of that ganglion, r. c., to the third spinal 



