But it is probable that interference between the nervous structures is not the sole cause of inhibition in the heart; we must look also to the relationship between nervous and muscular rhythms. Thus distension of the ventricle frequently diminishes or abolishes the action of the vagus, the stimulus which the pressure within the heart exerts on the muscular fibre appearing to more than counteract the inhibitory action of the nerve. The condition of the muscular fibre too is probably very important. Thus, feeding the frog's heart with a solution containing soda appears to paralyse the power of the vagus, which is again restored by potash.1 (Compare their action on the cardiac muscle, p. 307.)

It is indeed to an action on the muscle rather than on the nerve that we must probably look for the explanation of the action of atropine. For the heart in snails, though apparently destitute of both ganglia and nerves, is arrested by an interrupted current. This effect is prevented by atropine.

1 lowit, Pfluger's Archiv, xxv. p. 466.

Fig. 116.   Part of the posterior cardiac nerve, highly magnified, showing the ganglia.1

Fig. 116. - Part of the posterior cardiac nerve, highly magnified, showing the ganglia.1

Fig. 117.   Spiral ganglion cell from the pneumogastric of the frog. This figure is not taken from the cells in the cardiac nerves, as in them the connection between the spiral and straight fibres has not been clearly made out, but it is probable that these cells have a structure similar to the one figured (Ranvier, op.

Fig. 117. - Spiral ganglion cell from the pneumogastric of the frog. This figure is not taken from the cells in the cardiac nerves, as in them the connection between the spiral and straight fibres has not been clearly made out, but it is probable that these cells have a structure similar to the one figured (Ranvier, op. cit. pp. 114-120). a is the cell-body, n the nucleus, r the nucleolus, d nucleus of the capsule, f the straight fibre, g Henle's sheath, sp spiral fibre, g' its gaine, n' nucleus of Henle's sheath.2

1 Ranvier, Leqons d'Anatomie Generale, annee 1877-78, p. 106.

2 Ibid., p. 114.

It is exceedingly difficult, or perhaps impossible, with the physiological data which we at present possess, to give a complete and satisfactory explanation of the action of drugs on the heart, but it is evident that while all new discoveries tended for a while to render our ideas regarding the cardiac mechanism more and more complicated, our increasing knowledge now tends to render them more simple. Before long we may hope that systematic investigations into the action of drugs on the excitability, rhythm, and power to conduct stimuli of the cardiac muscle itself, on the action of drugs upon the rhythm of the ganglia, and on the rate of transmission by the nerves, as well as on the mutual relations of these various factors, will at last give us a clear understanding of this very difficult and complicated subject.