It was observed by Weber (i) that electric stimulation of the vagus nerve caused a slowing of the heart's rhythm, and if increased gave rise to a standstill of the heart in diastole; (2) that the heart beat gradually recommenced soon after the stimulus had been removed.

On the other hand (3) the section of both vagi produced an increase in the rapidity of the heart beat, varying according to the kind of animal experimented upon. Section of only one vagus, however, has not this effect.

From these experiments it would appear - 1. That some fibres of the vagus bear impulses of a checking or inhibitory nature to the intrinsic nerves of the heart. 2. That these influences are constantly in operation, or, in other words, the vagi exert a tonic inhibitory influence on the rapidity of the heart beat. 3. The tonic action of one vagus bears inhibitory influence sufficient to regulate the heart's action. This tonicity of the vagus inhibition is more marked in dogs and man than in rabbits, and is reduced to a minimum in frogs, where section of the vagi produces very little effect on the rate of the beat.

Tracing, showing the effect of weak Stimulation of Vagus Nerve.

Fig.123. Tracing, showing the effect of weak Stimulation of Vagus Nerve. Stimulus applied he tween vertical lines. (Recording surface moved from left to right).

Vagus inhibition is increased by the following circumstances - (a) certain psychical phenomena, such as terror, which may produce a temporary standstill; (b) deficiency of arterial blood in the medulla oblongata; (V) increase of the blood pressure within the cranium; and (d) reflexly by the stimulation of many afferent nerves, particularly those bearing impulses from the abdominal viscera to the medulla, and the afferent fibres of the opposite vagus.

The following drugs affect the cardiac nerve mechanisms: Muscarin produces diastolic standstill of the heart by exciting the local inhibitory ganglia or vagus terminals. Atropin causes quickening of the heart's action by paralyzing the endings of the vagus, and also those intrinsic mechanisms which are supposed to have an inhibitory effect. Nicotin produces at first a slowing of the heart by stimulating the inhibitory tone of the vagus. This is soon followed by exhaustion of the terminals and a consequent quickening of the heart beat. Large doses of curare paralyze the inhibitory fibres. Digitalis excites the vagus centre in the medulla, and thereby reduces the rapidity of the heart's beat.