THE FROG, AND ON THE ACTION OF THE VAGUS NERVE. 
1009 
of these curves taken as a whole it is seen that although stimulation of the vagus 
produces different effects at different times, these effects can all be reduced to the 
three following types :— 
1. Complete quiescence of both ventricle and auricles, followed by contractions 
which at first are scarcely visible, but which rapidly increase in size until at the maxi¬ 
mum they are much greater than before the stimulation of the nerve ; from this 
maximum they very gradually decrease until the original force of contraction is again 
reached (Plate 67, fig. 6, curve JB). 
2. During the stimulation no quiescence of either ventricle or auricles, but simply a 
diminution of the size of the contractions, followed by a rapid and marked augmenta¬ 
tion of the contraction curve beyond the original height, and then a slow gradual 
diminution to the size before the nerve was stimulated (Plate 67, fig. 6, curve A; 
Plate 68, figs. 8, 9, &c.). 
3. No primary diminution, but from the commencement of the stimulation the beats 
increase in force, and after a time gradually return again to the original size (Plate 68, 
fig. 10). Between these three types every conceivable variation may occur, so that a 
series of curves may be selected in which no line of demarcation can be drawn between 
complete primary quiescence, or to use the usual term, inhibition, on the one hand, and 
a simple primary augmentation of the contraction force on the other. Such a series is 
to a certain extent represented by Plates 67, 68, figs. 6, B, 6, A, 8, 9, 10. 
Further, the curves show that not only does the height of the contractions vary in 
a graduated series from the null point on the one side up to a height much greater than 
before the stimulation on the other, but also that this variation is independent of the 
rate of rhythm. In the majority of experiments the rhythm is seen to be decidedly 
accelerated by the stimulation of the nerve, in others no alteration of rhythm can be 
perceived. Thus in Plate 67, fig. 6, B, as soon as beats appear after the primary 
quiescence they show a slight acceleration of rhythm; so, too, in such figures as Plate 68, 
figs. 9, 10, where the nerve stimulation produced a primary diminution and a primary 
augmentation of the force of the contractions respectively, the rhythm is seen to be 
accelerated or unaltered from the commencement of the stimulation. 
In all cases where acceleration occurs, the return to the original rate is slow and 
gradual, in the same way as the return to the original contraction force. 
Another proof that this quiescence, which is caused by stimulation of the vagus, is 
not due to any interference with the rate of discharge from the motor ganglia, is most 
markedly shown by the fact that stimulation of the nerve often reduces the ventricle 
to a state of complete quiescence, while the auricles not only continue beating but beat 
with greater rapidity than before. Thus we may have complete quiescence of the ven¬ 
tricle with the auricles so nearly quiescent that their accelerated contractions are barely 
visible on the curve (Plate 67, fig. 7), in other cases with the auricular contractions 
more and more plainly visible (Plate 67, fig. 5), until finally we may obtain curves in 
which the ventricular beats -are diminished down to complete quiescence, while the 
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