When speaking in the following pages of the inhibitory action of the vagus on the heart I mean its power to affect the rhythm of the heart so as to render its pulsations slow or stop them entirely, and I do not include under the term inhibition, the power which the vagus also possesses of enfeebling the cardiac contractions, unless when this is expressly stated.

We distinguish between (a) stimulation of the vagus-roots by any cause whatever, and (b) stimulation of its ends in the heart2

1 Pflilger's Archiv, 1874, vol. viii., p. 606.

2 We use the term vagus-ends here for the sake of convenient distinction between the central cardio-inhibitory systems in the medulla oblongata and the peripheral one in the heart. A fuller explanation of the peripheral cardio-inhibitory apparatus will be given further on.

by dividing both vagi. Sometimes we inject the drug first, and see whether any slowing of the heart which it has produced disappears on section, or we may divide them before injecting the drug, and see whether any change, either in the way of slowing or acceleration, occurs after the injection. If the effect of a drug in slowing the heart is removed by dividing the vagi, we conclude that its action has been exerted on the vagus-roots: if it should still persist after their division, we conclude that it has acted on the vagus-ends in the heart or on the heart itself.

Thus aconitine,1 veratrine,2 erythrophloeum,3 and probably all members of the digitalis4 group stimulate the vagus-roots, so that the slowing of the pulse they produce is much lessened or completely abolished by section of the vagi, and takes place to a much less extent when the vagi are divided before the injection. That the slowing does not always completely disappear after section of the vagi, or is not always completely prevented by their previous section, is due to the fact that most of these drugs have also an action either on the ends of the vagus in the heart, or on the nervous mechanism or muscular fibre of the heart itself. Nicotine resembles the substances already mentioned in so far that the slowing which it would otherwise produce is somewhat lessened by section of the vagi, but only to a slight extent, its action being chiefly exerted on the peripheral cardio-inhibitory system.5 Physostigmine chiefly affects the heart itself, and so the slowing of the pulse it causes is not abolished by section of the vagi.6