Experimentalists have discovered another luminous fact connected with this influence of the brain upon the heart, namely, that although a current of a certain intensity (varying of course with the nature of the organism) will infallibly arrest the heart, if applied at once, yet if we begin with a feeble current and go on gradually increasing its intensity, we may at last surpass the degree which would have produced instantaneous arrest, and yet the heart will continue to beat energetically.
The effect of repetition in diminishing a stimulus is here very noticeable. It will serve to explain why, according to the traditions of familiar experience, we are careful to break the announcement of disastrous news, by intimating something much less calamitous, wherewith to produce the first shock, and then, when the heart has withstood that, we hope it may have energy to meet the more agitating emotions. The same fact will also serve, partly, to explain why from repetition the effect of smoking is no longer as it is at first to produce paleness, sweating, and sickness. The heart ceases to be sensibly affected by the stimulus.
Returning to the effects of a gentle stimulus, we can read therein the rationale of change of scene, especially of foreign travel, in restoring the exhausted energies. The gentle excitement of novel and pleasurable sights is not, as people generally suppose, merely a mental stimulus—a pleasure which passes away without a physical influence; on the contrary, it is inseparably connected with an increased activity of the circulation, and this brings with it an increased activity of all the processes of waste and repair. If the excitement and fatigue be not too great, even the sickly traveller finds himself stronger and happier, in spite of bad food, irregular hours, and many other conditions which at home would have enfeebled him. I have heard a very distinguished physician (Sir Henry Holland) say that such is his conviction of the beneficial influence of even slight nervous stimulus on the nutritive processes, that when the patient cannot have change of scene, change of room is of some advantage—nay, even change of furniture, if there cannot be change of room!
To those who have thoroughly grasped the principle of the indissoluble conjunction of nutrition and sensation, such effects are obvious deductions. They point to the great importance of pleasure as an element of effective life. They lead to the question whether much of the superior health of youth is not due to the greater amount of pleasurable excitement which life affords to young minds.
Certain it is that much of the marvellous activity of some old men, especially of men engaged in politics or in interesting professions, may be assigned to the greater stimulus given to their bodily functions by the pleasurable excitement of their minds. Men who vegetate sink prematurely into old age. The fervid wheels of life revolve upon excitement. If the excitement be too intense, the wheels take fire; but if the mental stimulus be simply pleasurable, it is eminently beneficial.
Every impression reacts on the circulation, a slight impression producing a slight acceleration, a powerful impression, producing an arrest more or less prolonged. The "shock" of a wound and the "pain" of an operation cause faintness, sometimes death. Indeed, it is useful to know that many severe operations are dangerous only because of the shock or pain, and can be performed with impunity if the patient first be rendered insensible by chloroform. On the other hand, the mere irritation of a nerve so as to produce severe pain will produce syncope or death in an animal which is very feeble or exhausted. It is possible to crush the whole of the upper part of the spinal chord (the medulla oblongata) without arresting the action of the heart, if the animal has been rendered insensible by chloroform; whereas without such precautions a very slight irritation of the medulla suffices to arrest the heart.
A moment's reflection will disclose the reason of the remarkable differences observed in human beings in the matter of sensitiveness. The stupid are stupid, not simply because their nervous development is below the average, but also because the connection between the two great central organs, brain and heart, is comparatively languid; the pneumogastric is not in them a ready channel for the discharge of nervous excitement. The sensitive are sensitive because in them the connection is rapid and easy. All nervous excitement must discharge itself through one or more channels; but what channels, will depend on the native and acquired tendencies of the organism. In highly sensitive animals a mere prick on the skin can be proved to affect the beating of the heart; but you may lacerate a reptile without sensibly affecting its pulse. In like manner, a pleasurable sight or a suggestive thought will quicken the pulse of an intelligent man, whereas his stupid brother may be the spectator of festal or solemn scenes and the auditor of noble eloquence with scarcely a change.
The highly sensitive organism is one in which the reactions of sensibility on the circulation, and of the circulation on the sensibility, are most direct and rapid. This is often the source of weakness and inefficiency—as we see in certain feminine natures of both sexes, wherein the excessive sensitiveness does not lie in an unusual development of the nervous centres, but in an unusual development of the direct connection between brain and heart. There are men and women of powerful brains in whom this rapid transmission of sensation to the heart is not observable; the nervous force discharges itself through other channels. There are men and women of small brains in whom "the irritability" is so great that almost every sensation transmits its agitating influence to the heart.