In order to prevent fallacies arising from stimulation of the vagus-roots by an asphyxial condition of the blood due to the action of the drug upon respiration, it is usual to maintain artificial respiration through a cannula placed in the trachea. This acts perfectly well in some cases, but if the drug should cause violent convulsive actions it may prevent the movements of the thorax occurring regularly, and therefore it is sometimes necessary to paralyse them by means of curare.

Moreover, it must be remembered that prolonged stoppage of the heart itself will allow the blood in the medulla to become venous and will thus irritate the vagus-roots. Prolonged arrest of the heart, therefore, tends by this action to prolong it still further, and functional inactivity tends to pass into death. This mechanism would render every intermission of the pulse very dangerous were it not that the same venous condition of the blood which stimulates the vagus-roots stimulates also the vaso-motor centre and the respiratory centre. The vaso-motor centre by contracting the arterioles maintains the blood-pressure during the prolonged diastole, and excitation of the respiratory centre tends to restore the arterial character of the blood. The venous condition of the blood also stimulates accelerating centres in the medulla (Dastre and Morat).

Stimulation of the Heart by increased Blood-pressure. It has already been mentioned that increased blood-pressure usually renders the beats of the heart slower by the stimulating action it exerts on the vagus-roots. When the vagi are divided, however, its effect is usually quite different, and a rise in blood-pressure after division of the vagi renders the pulse quicker instead of slower, at least generally. An opposite result has been found by Marey in the heart of the tortoise, where increased pressure rendered the beats slower. The reason of the difference observed between the mammalian heart and that of the tortoise is probably due to the different development of the nervous and muscular structures. The tortoise heart acts more like a single simple muscle, and the more resistance it has to overcome the more slowly does it work.

In the mammalian heart the increased pressure appears to stimulate the nerves, so that the more resistance it has to overcome the more quickly does it work - that is, if the vagi have been cut. The sensibility of the nervous system in the heart to increased pressure appears to be diminished by atropine, for Schiffl has found that a quantity of this poison slightly larger than will dilate the pupil lessens the sensibility of the heart to changes in blood-pressure so much that the pressure may be first increased to three times the normal and then diminished to one-half, or even one-third, without any change in the pulse-rate being produced.

1 La Natione, 1872, No. 235.

Such an observation suggests that atropine would be useful in lessening pain or palpitation of the heart in persons with high blood-pressure or suffering from the effects of cardiac strain consequent on violent muscular exertion. I have tried it in such cases sometimes with apparently great benefit, at other times with little result. The cases of failure may, however, have been due to the remedy not being pushed far enough, as in them the pupil was not markedly dilated.