THE FROG, AND ON THE ACTION OF THE VAGUS NERVE. 
L013 
In the first part of this paper I have attempted to prove that the ventricle beats 
synchronously with the auricles when a due relation subsists between the strength ol 
the impulses from the motor ganglia and the excitability of the ventricle. When, 
however, the strength of the impulses is diminished either by clamping or by the 
action of heat upon the motor ganglia, this due relation is destroyed and the ventricle 
either beats synchronously with .every second, third, or more contractions of the 
auricles or else does not beat at all. If under these circumstances the excitability of 
the ventricular muscle be increased, then the relation between the strength of the 
impulses and the muscular excitability, which is necessary for synchronous contractions, 
will be restored, either fully or partially, according to the extent to which the 
excitability is raised. On the other hand, if the excitability be lowered then that due 
relation will be still further destroyed, and each beat of the ventricle will follow upon 
a still larger number of auricular beats than before. 
In accordance with this method of testing the excitability, experiment proves that 
the vagus may cause a diminution of the excitability of the ventricle during the first 
part of its action, during, therefore, the period when it is known to diminish the force 
of the contractions ; and may cause an increase of excitability when it augments the 
force of the contractions. 
I will now proceed to give the facts upon which the truth of this proposition 
depends, and will first examine the action of the nerve when the clamp in the auriculo- 
ventricular groove is tightened so as to make the ventricle beat synchronously with 
every second or third beat of the auricles. 
In this case the most striking effect of stimulation of the nerve is the alteration 
which is produced in the rate of the ventricular rhythm rather than the variations in 
the force of the ventricular contractions. It is true that the nerve stimulation may 
still cause the force of the ventricular contractions to be diminished and augmented 
in the same way as already described, but neither the diminution nor the augmentation 
are so manifest as when the ventricle is beating synchronously with every auricular 
beat; this is doubtless largely owing to the greater size of the ventricular contractions 
in consequence of their slower rate of rhythm. This is, however, not the main 
distinction between the two cases. 
When the ventricle is beating synchronously with every beat of the auricles we have 
seen that this synchronism continues during the period of diminution as well as during 
the period of augmentation of the force of the contractions ; when, however, the 
ventricle is beating synchronously with every second beat of the auricles the same law 
does not necessarily hold; in many cases stimulation of the nerve causes a sudden 
alteration in the relation of the ventricular to the auricular rhythm. 
In the first place the rhythm of the ventricle becomes synchronous with that of the 
auricles for a longer or shorter period. This period of synchronism corresponds to the 
time when the contractions would have been greatest if the ventricle had been beating 
in due sequence with the auricles ; it may therefore occur sometimes during the stimula- 
