THE FROG, AND ON THE ACTION OF THE VAGUS NERVE. 
995 
means of a drum tlie two levers write at the same moment upon the blackened paper, and the upper 
lever moving downwards records the contractions of the auricle, while the lower lever moving upwards 
traces those of the ventricle. 
The vagus nerve is now laid upon a pair of platinum electrodes E, and kept moist by laying over it a 
piece of filter paper dipped into normal saline solution. The electrodes are in connexion with an ordinary 
induction coil driven by a single Daniell cell, the interruptor of which is a tuning-fork instead of the 
usual hammer. In addition to the two tracings described above, the usual time and stimulation markers 
write on the blackened paper. 
In the description just given 1 have spoken of the clamp as being placed in the 
auriculo-ventricular groove, so as to register the auricular and ventricular contractions 
simultaneously; it can, however, be placed across any portion of the heart, for example 
between the sinus and auricles, so as to register the sinus and auricular contractions ; 
midway between the base and the apex of the ventricle, so as to register the contrac¬ 
tions of the base and apex simultaneously, &c. 
Besides the advantage obtained by the power of registering at will the contractions 
of any two contiguous parts of the heart, the clamp enables us to study the effects of 
compression at different points much more delicately than the old plan of ligaturing, for 
it is possible either just to hold the tissue so as not to injure it physiologically or to com¬ 
press it up to any required amount by the simple movement of the micrometer screw. 
Further, it is possible to cut the ventricle open from base to apex, and to apply 
poisons or fluids of any kind to that part of the heart which is situated below the 
clamp without any chance of their reaching the parts above the clamp (the latter being 
provided with a flange on each side to prevent the fluids passing over it). Also the 
parts on the one side of the clamp can be heated or cooled independently of the parts 
on the other side, and under all these different conditions the effect of stimulation of 
the vagus nerve can be studied. Throughout the course of each experiment the heart 
is kept moist by the application of salt solution. 
PART I. 
On the Rhythm of the Heart. 
If we imagine a portion of muscle made to contract rhythmically by the action of 
certain cells situated at a distance from it, then it is clear that the rate of rhythm will 
be independent of any increase in the excitability of the muscle, if the rhythm is due 
to separate impulses passing from the cells to the muscles, each of which is able to 
cause a contraction ; on the other hand, an increase in the muscular excitability must 
cause a quicker rate of rhythm, if the rhythm is dependent upon some process of 
summation of stimuli taking place in the muscle itself. 
Now, by the method described above it is possible to influence in different ways 
either the ventricle alone on the one side of the clamp without affecting the sinus and 
auricles on the other side, or only the auricles and other parts on that side of the 
