146 A MANUAL OF PHYSIOLOGY 



exposed (Fig. 61), excited much speculation before it was known 

 that sympathetic fibres existed in the vagus. There is no longer 

 any doubt that it is due to the stimulation of these accelerator 

 or, as it is better to call them (since mere acceleration is not the 

 only consequence of their stimulation), augmentor fibres in the 

 mixed nerve. For (i) excitation of the roots of the vagus proper 

 within the skull, and therefore above the junction of the sympa- 

 thetic fibres, causes no secondary augmentation, or very little, 

 and the inhibition lasts far longer than when the mixed trunk is 

 stimulated. (2) Excitation of the upper or cephalic end of the 

 sympathetic cord before it has joined the vagus causes, after a 

 relatively long latent period, marked augmentation. And if the 

 contractions of the heart are registered, the tracing bears a close 

 resemblance to the curve of secondary augmentation following 

 excitation of the mixed nerve on the other side with an equally 

 strong stimulus and for an equal time. (3) When the vago-sym- 

 pathetic is stimulated weakly there is little or no secondary aug- 

 mentation. Now, it is known that the augmentor fibres require 

 a comparatively strong stimulus to cause any effect when they 

 are separately excited, whereas a weak stimulus will excite the 

 inhibitory fibres. 



The question arises at this point, why it is that, when the inhibi- 

 tory and augmentor fibres are stimulated together in the mixed 

 nerve (and the same is true when the sympathetic on one side and 

 the vagus on the other are stimulated at the same time), the inhibi- 

 tory effect always comes first, when there is any inhibitory effect, 

 while the augmentation always has to follow. The answer has 

 sometimes been given, that the latent period of the augmentor fibres 

 is longer than that of the inhibitory fibres. But although this is 

 certainly the case, the answer is insufficient. For the period of post- 

 ponement may be much greater than the latent period of the sym- 

 pathetic fibres when stimulated by themselves. The inhibition 

 apparently runs its course without being affected by the simultaneous 

 augmentor effect, which, lying latent until the end of the inhibition, 

 then bursts out and completes its own curve. It is not like the 

 passing of two waves through each other, but rather like the stopping 

 of one wave until the other has passed by. It seems as if augmenta- 

 tion cannot develop itself in the presence of inhibition at least, 

 until the latter is nearly spent. Like a musical-box devised to play 

 a series of melodies in a fixed order, and from which a particular 

 tune cannot be obtained till those preceding it have been run 

 through, the heart, in some way or other, is arranged, in the presence 

 of competing impulses from its extrinsic nerves, to play the tune of 

 inhibition before the tune of augmentation. In the frog, at any 

 rate, the two processes can hardly be considered as antagonistic, in 

 the sense that a definite amount of augmentor excitation can over- 

 come a definite amount of inhibitory excitation. Nor is it the case 

 that when the heart is played upon at the same time by impulses of 

 both kinds, it pits them against each other and strikes the balance 

 accurately between them. It is possible, however, that when the in- 

 hibitory fibres are very weakly, and the augmentor fibres very strongly 

 stimulated, the amount of inhibition may be somewhat diminished . In 



