THE FROG, AND OH THE ACTION OF THE VAGUS NERVE. 
1019 
On the action of the vagus nerve upon the tonicity of the heart. 
The question whether any particular influence upon the heart causes the relaxation 
of the tissue to become greater or less than the normal, is always attended with the 
difficulty that it is impossible to say what the normal amount of relaxation really is. 
Thus with the ordinary rate of rhythm the amount of relaxation is always greater with 
a slower beat than with a quicker one, a fact which is to be explained by the simple 
supposition that the condition of contraction has not absolutely passed away when the 
next contraction takes place unless the contractions follow at a slower rate than the 
normal; so that for this reason alone a greater relaxation must occur if the vagus causes 
a slowing of the beats and a less relaxation if the beats are made more frequent. On 
the other hand, if the tissue is relaxed when the rhythm of the beats is accelerated or 
unaltered, we may then fairly speak of an increased relaxation of the tissue, and if the 
relaxation is less, when the rate is the same or slower, then we may speak of a 
diminished relaxation of the tissue. Thus, for instance, every experiment shows that 
muscarin and lactic acid truly increase, while sodium hydrate and digitalin truly diminish 
the normal relaxation of the tissue. 
Now in endeavouring to estimate the extent of relaxation in a hollow muscle like the 
ventricle, it is clear that the measurement of its capacity will afford a better chance of 
obtaining a visible amount of relaxation than any mere linear measurement such as 
is given by the method described above. I have therefore endeavoured to find out 
whether the vagus really causes any relaxation .of the ventricle by the following 
method :— 
The instrument which I have used is a modification of Roy’s tonometer* and is represented in Plate 70, 
figs. 22, 23. The glass chamber A is in connexion by means of a metal tube with the recording chamber B, 
to the lower end of which the membrane used by Roy is tied and within which the piston attached to this 
membrane is able to move up and down and so to cause corresponding movements of the lever. The 
funnel and tap on the top of the metal chamber B is for the purpose of filling this chamber and the con¬ 
necting tube 0 with oil. The tap in the tube C enables us to free this tube from air; and somewhat below 
the middle of the chamber B is a tap which allows oil to be drawn out of the apparatus by means of the 
indiarubber tube D, and so to exhaust the pressure within the whole apparatus and raise the lever to the 
horizontal position as described by Roy in the original description of his tonometer. The glass chamber A 
is provided with a vulcanite stopper which fits absolutely tight, so that any variation in the contents of 
the chamber A must be transmitted to the chamber B through the tube C, and cause a corresponding 
variation in the position of the lever. There is no necessity that the chamber A should be filled with oil; 
it may be filled with normal salt solution or any other fluid without preventing the working of the mem¬ 
brane, because the lower extremity of the chamber B is on the same level with the top of the chamber A, 
and therefore the oil in the chamber B will not be contaminated by the salt solution in A. 
The vulcanite stopper, of which a separate drawing is given on Plate 70, fig. 23, contains five holes of exactly 
the same diameter; small portions of a piece of metal tubing which has been ground accurately to fit these 
holes can be inserted into one or more of them, according to the requirements of each experiment, and those 
* Journal of Physiol., vol. i., p. 452. 
