i6o THE CIRCULATION OF THE BLOOD AND LYMPH 



them (since mere acceleration is not the only consequence of their 

 stimulation), augmentor fibres in the mixed nerve. For (i) excita- 

 tion of the roots of the vagus proper within the skull, and therefore 

 above the junction of the sympathetic 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 augmenta- 

 tion. And if the contractions of the heart are registered, the 

 tracing bears a close resemblance to the curve of secondary augmen- 

 tation following excitation of the mixed nerve on the other side 

 with an equally strong stimulus and for an equal time. (3) When 

 the vago-sympathetic is stimulated weakly there is little or no 



Fig. 72. Frog's Heart: Vagus stimulated. Temperature of heart 8 C.; 78 mm. 

 between the coils. Diminution in force of auricle and ventricle, but not com 

 plete standstill. Time -tracing shows two-second intervals. 



secondary augmentation. 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 inhibitory 

 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 inhibitory 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 insuffi- 

 cient. For the period of postponement may be much greater than the 



