Stimulation of either set of fibres before they are mixed together produces effects typical of one or the other only. In the dog, whose normal heart rate is about i oo beats per minute, stimulation of the accelerator nerves may increase it to 26o beats, and at the same time the strength of each contraction will be increased. The latter effect is most marked in hearts which have begun to fail and which are therefore dilated.
The question whether, under normal conditions, the accelerator fibres are in a state of tone like that of the inhibitory fibres of the vagus is not definitely settled. It is customary to assume the existence of such a tone, but to consider it as less pronounced than that of the vagus. In most experiments, extirpation of the Stellate ganglia leads to a retardation of the heart. Many observers have shown that the tonic conditions of the two centres stand in a reciprocal relation to one another. Whenever the tone of the inhibitory fibres is increased, that of the accelerator fibres is diminished ; thus the final effect on the heart will be an algebraical summation of the two influences. The tone of the accelerator centre is greatly increased in asphyxia, in cerebral anaemia, in the case of various sensory stimuli (especially painful stimuli), and probably in the case of muscular exercise. In the first two states, which are pathological, the tone of both nerves is in creased simultaneously. Thus, in the absence of the sympathetic nerves, asphyxia produces retardation of the heart by stimulating the vagus, and in the absence of the vagus it produces an accelera tion. In the presence of both nerves, it first retards the heart and may even arrest it until the vagi become paralysed, of ter which the heart greatly accelerates above the normal, because of the co-existent stimulation of the sympathetic centre.
The Suprarenal Gland.—It has already been mentioned that the influence of the vagus can be modified by stimuli affecting the centre, or by substances which stimulate or paralyse the periph eral nerve endings of the vagus within the heart itself. In the case of the sympathetic innervation, the organism normally produces a chemical substance which stimu lates all the sympathetic nerve endings.
The production of this substance is the function of the supra renal (or adrenal) gland, and the substance has not only been obtained in a chemically pure state, but has also been synthesised.
It is known as adrenaline or suprarenin. Adrenaline is active in very small amounts, concentrations of one in one hundred millions producing a strongly exciting effect. Under normal conditions,
however, the quantities entering the blood are probably too small to have any physiological effect. The liberation of adrenaline is under the influence of the splanchnic nerves, section of which diminishes the secretion, while stimulation greatly increases it. Certain drugs, asphyxia, various emotions such as fear or anger lead to an excessive production of adrenaline. Directly the adrena line reaches the heart, even when its connections with the nervous system are all severed, the contractions become considerably faster and extremely energetic, and the heart becomes able to cope with a greater strain (either in the shape of arterial resistance or increased venous inflow) than it could do without the stimulus of adrenaline.
To summarize, we may state that the heart beat is a property of the heart itself, and as such it is independent of the nervous - system. The latter may however be said to keep the activity of the heart under constant supervision, moderating its action by means of the vagus, and increasing it by means of the sympathetic nerves, thus adapting it to the general requirements of the body. Under conditions of great physical strain, the organism is able to make use of its store of adrenaline, which assists the nervous regulation of the heart beat. Thus, even after complete denerva tion of the heart, a certain amount of adaptation of its activity is still possible.