When the vagi are stimulated by an induced current, the heart usually stops in diastole.

Fig. 105.   Tracing of the pulsations of a ventricle separated from the auricles by section at the auriculo ventricular groove. After Ranvier, Lecons, 1877 78.

Fig. 105. - Tracing of the pulsations of a ventricle separated from the auricles by section at the auriculo-ventricular groove. After Ranvier, Lecons, 1877-78.

The effect of stimulation may be observed either on the heart simply exposed or by means of Ludwig and Coats' apparatus. The action of both vagi is not always alike. The right vagus has usually a greater power to arrest the heart than the left. The action of the vagus varies also according to the condition of the heart, and may produce different effects. It may cause, 1st, stoppage of the heart's beats, followed after an interval by slow pulsations or by small rapid pulsations, gradually becoming larger and stronger; 2nd, it may cause them to become small and slow without actual stoppage - this is the usual effect of irritation of the vagus in the living body; 3rd, it may cause the pulsations to become simply small and rapid without any stoppage; 4th, it may cause them to become rapid; 5th, it may cause them to become more powerful (Figs. 112 to 115, p. 324).

It may also act differently on the auricles and ventricle, producing still-stand of the ventricle and rapid pulsation of the auricles. These differences are probably due to a great extent to the vagus of the frog being really the combined vagus and sympathetic. At present the chief point upon which I wish to insist is that irritation of the vagus usually causes still-stand of the heart.

When the venous sinus is stimulated, still-stand of the heart is produced, which is even more complete and permanent than that which follows irritation of the vagus.