THE FROG, AND ON THE ACTION OF THE VAGUS NERVE. 
999 
impulse, because each impulse, though unable to cause a contraction of the muscle, may 
increase its excitability to the height necessary for the production of a beat by the time 
the next impulse reaches the tissue. 
The proof of these assertions depends upon the fact that, by the method of clamping, 
it is possible to alter the relation between the strength of the impulses and the 
excitability of the ventricular muscle without affecting the relation between the motor 
ganglia and the auricular muscle. The rate, therefore, at which the impulses are sent 
out from the motor ganglia is indicated by the rate of the auricular beats, and the 
extent of synchronism between them and the beats of the ventricle determines the 
number of impulses that have been made inefficient, as far as the causation of a 
ventricular beat is concerned, by the action of each special operation. 
This want of sequence between the auricular and ventricular contractions can be 
obtained by three different methods. 
1. By tightening the clamp in the auriculo-ventricular groove. 
2. By heating the auricles and sinus without heating the ventricle. 
3. By the application of various poisons to the ventricle alone. 
By the use of the micrometer screw attached to the clamp it is possible to compress 
the tissue so slightly, and yet to hold it with sufficient firmness, that the sequence of 
ventricular upon auricular beats continues without alteration or interruption; it is 
possible also to compress it so strongly that the motor impulses can no longer reach 
the ventricle, and therefore the ventricle remains quiescent, while the auricles continue 
beating with unaltered rhythm ; and it is possible to compress it to any extent inter¬ 
mediate between these two extremes, and to observe the alteration in the ventricular 
rhythm so produced. This latter case is that with which we are specially concerned at 
present, and in every instance I have found that with gradual tightening of the clamp 
the ventricle does not pass abruptly into a state of quiescence, but that the increased 
compression always causes the ventricle to beat slower than the auricles. This slower 
rate of beating is never independent of the auricular rate, but is connected with it in 
such a way that the ventricle, instead of beating synchronously with each beat of the 
auricles, beats synchronously with every second, third, fourth, or more auricular beat. 
The commonest effect by far, and the one which is most permanent, is that in which the 
ventricle beats synchronously with every second beat of the auricles ; and, indeed, by 
careful manipulation of the micrometer screw, it is always possible to bring about this 
particular kind of rhythm, and in many cases to make the ventricle beat permanently 
with this rhythm for the rest of the experiment. In other cases this rhythm may last 
for a considerable length of time, and then the ventricle return to a rate synchronous 
with that of the auricles. So, too, when the clamp is first tightened the half-rhythm 
may be caused immediately, or the ventricle may cease to beat for a short time, and 
then commence to beat with intervals between the beats, corresponding to a large 
number of auricular beats, such as 1 ventricular beat to 6 auricular ; quickly passing 
mdccclxxxii. 6 u 
