In spite of these evidences of a temporary independence of brain and heart, as individual organs, there is nothing more certain than the intimate interdependence of the sensitive and circulating systems; and if in lower animals the interdependence of the two great central organs is less energetic than in the higher, the law of the intervention of sensibility in all processes of nutrition is unaffected. In fact, wherever the motor mechanism is muscular, as it is in all but the simplest animals, the necessary intervention of sensibility is an à priori axiom. Every action in the organs of such animals is a manifestation of muscular contractility, and there is no known means of exciting this contractility except by the stimulus of a nerve.
The heart is a muscle. Some years ago there was a school of physiologists advocating the hypothesis that the action of the heart was due to the irritability of its muscular tissue, which was stimulated by the presence of blood. The great Haller was the head of this school, and his "Memoires sur la nature sensible et irritable des parties" [Footnote 125] is still worthy the attention of experimentalists. And, indeed, when men saw the heart continue its pulsations some time after death, and even after removal from the body, and saw, moreover, that after pulsation had ceased it could be revived by the injection of warm blood, there seemed the strongest arguments in favor of the hypothesis. Unhappily for the hypothesis, the heart continues to beat long after all the blood has been pumped out of it, consequently its beating cannot be due to the stimulus of the blood.
[Footnote 125: Lausanne, 1756, in 4 vols. ]
In our own day the difficulty has to a considerable extent been removed by the discovery of a small nervous system specially allotted to the heart,—nerves and ganglia imbedded in its substance, which there do the work of nerves and ganglia everywhere else. Cut the heart into pieces, and each piece containing a ganglion will beat as before; the other pieces will be still. Beside this special cardiac system which influences the regular pulsations, there is the general nervous system, which accelerates and arrests these pulsations at every moment of our lives. The heart is thus connected with the general organism through the intervention of the great sensory apparatus. Filaments of what are called the pneumogastric nerves connect the heart with the spinal chord and cerebral masses; but it is not the influence of these filaments which causes the regular beatings of the heart (as physiologists formerly supposed), and the proof is that these filaments may all be cut, thus entirely isolating the heart from all connection with the great nervous centres, and yet the heart will continue tranquilly beating. What causes this? Obviously the stimulus comes from the heart's own nerves; and these are, presumably, excited by the molecular changes going on within it.
Physiologists, as we said just now, supposed that the filaments of the pneumogastric nerves distributed to the heart caused its beating. What then was their surprise, a few years since, when Weber announced that the stimulation of these fibres, instead of accelerating the heart's action, arrested it! Here was a paradox. All other muscles, it was said (but erroneously said), are excited to increased action when their nerves are stimulated, and here is a muscle which is paralyzed by the stimulation of its nerves. The fact was indisputable; an electric current passed through the pneumogastric did suddenly and invariably arrest the heart. Physiologists were interested. The frogs and rabbits of Europe had a bad time of it, called upon to answer categorically such questions put to their hearts. In a little while it appeared that although a strong electric current arrested the pulsations—and in mammals instantaneously—yet a feeble current accelerated instead of arresting them. The same opposite results followed a powerful and a gentle excitation of the upper region of the spinal chord.
To these very important and suggestive facts, which throw a strong light on many phenomena hitherto obscure, let us add the interesting facts that in a healthy, vigorous animal, the heart quickly recovers its normal activity after the withdrawal of the electric stimulus; but in a sickly or highly sensitive animal the arrest is final.
Who does not read here the physiological explanation of the familiar fact that powerful mental shocks momently arrest the heart, and sometimes arrest it for ever? That which a powerful current will do applied to the pneumogastric nerve, will be done by a profound agitation of grief or joy—truly called a heart-shaking influence. The agitation of the great centres of thought is communicated to the spinal chord, and from it to the nerves which issue to various parts of the body: the limbs are violently moved, the glands are excited to increased activity, the tears flow, the facial muscles contract, the chest expands, laughter or sobs, dances of delight and shouts of joy, these and the manifold expressions of an agitated emotion, are the after results—the first effect is an arrest, more or less fugitive, followed by an increase of the heart's action. If the organism be vigorous, the effect of a powerful emotion is a sudden paleness, indicating a momentary arrest of the heart. This may be but for an instant; the heart pauses, and the lungs pause with it—"the breath is taken away." This is succeeded by an energetic palpitation; the lungs expand, the blood rushes to face and brain with increased force. Should the organism be sickly or highly sensitive, the arrest is of longer duration, and fainting, more or less prolonged, is the result. In a very sensitive or very sickly organism the arrest is final. The shock of joy and the shock of grief have both been known to kill.
The effects of a gentle stimulus we may expect to be very different, since we know that a feeble electric current stimulates the heart's action. The nature of the stimulus is always the same, no matter on what occasion it arises. It may arise from a dash of cold water on the face—as we see in the revival of the heart's action when we throw water on the face of a fainting person. It may arise from inhaling an irritant odor. It may arise from the pleasurable sight of a dear friend, or the thrill of delight at the new birth of an idea. In every case the brain is excited, either through an impression on a sensitive nerve, or through the impulses of thought; and the sensibility thus called into action necessarily discharges itself through one or more of the easiest channels; and among the easiest is that of the pneumogastric nerve. But the heart thus acted on in turn reacts. Its increased energy throws more blood into the brain, which draws its sustaining power from the blood.