1. Notwithstanding what has been said of the contractility of muscles being independent of any influence continually flowing to them from the brain or spinal cord, it is well ascer tained that in a living and entire animal, where all the functions of the body are, for wise and important purposes, made liable to change, from changes in the nervous system, the con tractile power of various moving parts is sub ject to increase or diminution from physical causes acting in these larger masses of the ner vous system, just as they are from various acts and affections of Mind, the effects of which may be said to be iniitated by those physical causes. Thus in the experiments of Le Gallois, of Dr. Wilson Philip, of Flourens, and others, suddenly crushing any large portion, either of the brain or spinal cord, has been found uni formly to depress or even extinguish the power of the heart ; the well-known fatal effect often ob served from sudden violent injury of the epi gastrium in the human body, has been ascribed with probability to the injury of the great semi lunar ganglion ; and the depression of the heart's action which attends Concussion, and which is the immediate cause of death in the most quickly fatal cases of that kind, is also generally regarded as an impression, made ori ginally on the nervous system, and immediately transmitted to the heart. On the other hand, slighter and more continued physical irritations of the nervous system appeared in many expe riments, especially of Dr. Wilson Philip, to augment the irritability of the heart. It is true that, in all these cases, some have supposed the effects of the violence to be on the organs of circulation directly, and not through the inter vention of the nerves; but when it is remem bered, that some of those injuries, which are the most rapidly fatal to the heart's actions, (such as the pushing of a probe along the spinal canal,) do not necessarily imply any great violence to the body at large ; and further, that precisely similar effects on the heart's ac tion (both increase and diminution) often result from mental emotions and passions, which cer tainly act first on the nervous system, the ac count which we give of the mode of action of these causes appears to be sufficiently con firmed.
One cause, acting primarily on the nervous system, which seems to have a peculiar de pressing effect on the heart's action, is, sudden re moval of the pressure to which the brain had previously been subjected. The effect of this on the heart has been repeatedly seen in surgi cal operations ; and this seems to be an essen tial part of the pathology of seveial cases of syncope, particularly of that which results, either from bloodletting in the erect posture, or from tapping in ascites.
It is very remarkable that the heart, which is so strictly an involuntary muscle,- and so little liable to excitation by stimuli applied to its nerves, is much more liable than the voluntary muscles both to sudden increase and to diminution, or even total loss, of vital power from such causes as we have now considered. But a little reflection will shew, that the direct stimulation of a muscle, and the increase or diminution of its irritability, are perfectly distinct cases. And we may approxi mate, at least, to an explanation of the peculiar liability of the heart (and probably of other involuntary muscles) to the influence of such causes acting through the nervous system, as augment or depress the vital power, when we remember two 'facts : 1. that the causes which
act in this way are very generally such as are applied to large portions of the brain or spinal cord ;* and 2. that the arrangements of the ganglionic nerves are such as to place the heart and other organs supplied from the ganglia, in connexion with the whole extent of the cerebro spinal axis, and hudly with anyindividual part of it more than another.
2. There are various external agents, by the application of which the vital power of con tractile parts, and especially of the heart,—the main agent in the circulation,—may be altered or even destroyed. It is increased, not only by moderate increase of the Temperature in which living parts are kept, and of the quantity of arterial blood sent to them, but also by Elec tricity applied in a low degree of intensity, and by various articles of diet and medicinal agents, such as the various preparations of Alcohol ; and it is diminished, or even suddenly extin guished often by the same agents applied in excess, (as in the case of Lightning when most rapidly fatal,) and still more remarkably by certain Poisons, such as the upas antiar, tobacco, digitalis, arsenic, and hydrocyanic acid. It is still doubtful throUgh what medium these poi sons act on the vital power of the heart ; but it is certain that the effect which they produce on that power is the immediate cause of the death resulting from them.t In cases of the most sudden death produced by such causes acting in the utmost inten sity, the contractile power in the voluntary muscles, as well as in the heart, has been found to be very much diminished or even nearly ex tinguished ; and it is very important to observe, that in such cases the property of coagulation in the blood is likewise lost ; which seems clearly to indicate (what various other. facts confirm) that this change in the blood is de pendent on the existence in that fluid of a certain degree of the same vital properties, to which. we give the name of Contractility as ex isting in the solids.
3. The contractile power of living parts is liable to much alteration from the degree in which it is itself exercised. The immediate effect of frequently repeated stimulation of a voluntary muscle, whether by physical or mental stimuli, in a living or newly killed animal, is gradual diminution or ultimate extinction, or what is usually called Exhaustion of its Irri tability ; which is gradually restored when the stimulation is discontinued and the muscle is at rest.
But the theoretical conclusions which have been drawn from this fact have greatly exceeded the legitimate inferences. It is by no means clear that such increased action of involuntary muscles, as results from causes of the kinds just mentioned, which exalt or increase their contractile power, is necessarily followed by any corresponding depression. On the con trary, in the case of violent exercise, in many instances of mental agitation and excitement, and in the course of certain febrile and inflam matory diseases, we see the heart's action greatly .and permanently increased, without evidence of any subsequent loss of power which can reasonably be ascribed merely to the cir cumstance of increased action.