THE FROG, AND ON THE ACTION OF THE VAGUS NERVE. 
1017 
On the action of the vagus upon the ventricular muscle when that muscle is heating with 
alternately strong and ivea/c heats. 
When the heart is suspended in the manner already described and the ventricle is 
beating in due sequence with the auricles, it frequently happens that, although the 
auricular contractions are all equal in force, the ventricle is seen to be beating with 
alternately weak and strong contractions, as is shown in Plates 67, 69, figs. 7, 19, 21. 
If, now, the vagus be stimulated, this alternation in the size of the ventricular contrac¬ 
tions disappears simultaneously with the increase in the force of the contractions caused 
by the nerve stimulation, and reappears again as the contractions return to their 
original size; and if the nerve be stimulated a second time, while the contractions are 
still equal in force, then with the primary diminution in force caused by the stimula¬ 
tion the alternately weak and strong beats again appear, and again give way to beats 
of equal force with the subsequent augmentation of the force of the contractions. This 
is well shown in Plate 69, fig. 19. 
It is difficult to come to any final conclusion upon the causation of this alternation 
in the strength of the ventricular contractions, though it is possible to somewhat limit 
the area of discussion. In the first place, they are clearly local in origin, for by clamp¬ 
ing across the middle of the ventricle the same alternation in strength is sometimes 
seen in the beats of the apex, while those of the base of the ventricle remain of the 
same strength throughout (Plate 69, fig. 20). Here, too, the vagus removes this 
alternation in the same way as in the case of the whole ventricle. Also, as far as I 
have seen, this phenomenon is confined to the ventricle. I have never seen any 
appearance of this alternation of contraction-force in the auricular beats. In the second 
place, they are not necessarily due to the clamp, for the same alternation may occur 
when the ventricle alone is suspended and no clamp is used. 
Further, the sizes of these alternately weak and strong contractions are clearly 
dependent upon each other, so that, other things being equal, the more marked the 
diminution in the weak contractions the more marked also is the increase in the strong 
ones which alternate with them; therefore if, as sometimes happens, the alternation 
disappears, temporarily though it may be, when the ventricle is moistened with normal 
saline solution, then the size of the equal contractions is intermediate between the 
size of the alternately weak and strong ones. This relation is beautifully seen when, 
as is apt to occur, a secondary rhythm appears in these contractions, as is shown in 
Plate 69, fig. 21. Here it is seen that the weak contractions increase to a maximum 
while the strong contractions diminish simultaneously to a minimum, and vice versa, the 
minima of the weak correspond to the maxima of the strong. 
Now we know from the experiments of Bowditch* that the force of the ventricular 
contractions is independent of the strength of the stimulus. The explanation, there¬ 
fore, of this alternation in the force of the contractions must be sought for in the 
* Op. ait. 
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