In animals incompletely developed there is no emotion capable of modifying the rhythm of the heart. In a series of experiments which I made on the heart in a hen’s egg during the first days of its development, I found that the application of the strongest inductive currents, such as were unbearable on the hands, produced not the least effect. It was a strange sight, this surprising tenacity and unexpected resistance in a little heart which was scarcely visible, and which pulsated tranquilly under electrical discharges which would have killed instantaneously the heart of a horse or an ox.
This shows us how well the organs are adapted to their functions. It is the task of the heart in the chicken to work blindly and incessantly in order to bring into circulation the little particles which gradually build up the body of the animal, using for this purpose the materials accumulated in the egg after it has received the spark of life through fecundation. In the embryo there is no need to receive the impressions of the outside world, and the organs for this purpose are still lacking, the nerves have not yet appeared, the heart is free in the midst of the chaos of matter in the course of organisation.
III
The fully-developed heart has a much more complicated innervation than the other muscles. The arm or leg cut off from the body ceases at once to move, but the heart removed from an animal continues to beat for a long time. Those who frequent the anatomical lecture-rooms notice sometimes with surprise slight movements at the base of the heart, although the rest of the corpse has been cold and motionless for a whole day. The heart owes this tenacity of life to the structure of its thin walls, to its being immersed in blood, and, more than all, to there being in its flesh little nerve-centres called ganglia. It is on this account, however, by no means independent of the brain and spinal cord, which can modify the rhythm and force of its pulsations according to the needs of the internal economy. Our organism is one of the most wonderful examples of that happy autonomy where liberty and the functions of each organ are always subordinated to the interest and advantage of all others, while the joint administration has as its object the maintenance of life and the welfare of all.
The centre of the cardiac nerves is in the medulla oblongata, in the most important part of the nervous system, near that point the wounding of which even with a pin-point causes instant death, because there all the paths of the nervous system converge.
Of the two nerves which carry commands to the heart, one serves principally to slacken the pulsations, and has, since it acts as a brake, received the name of inhibitory nerve; while the other, serving to increase the frequency of the beats, to spur them on, so to speak, is called the accelerator nerve.
The functions of the cardiac nerves, which may seem in this way to be extremely simple, are in reality very complicated. Galvani was the first who showed that an irritation of the spinal cord brought about an arrest, or, as he called it, an enchantment (incantesimo) of the heart.
IV
Boccaccio describes in a masterly manner the effects and changes which love produces on the pulse:
'Thus it happened that he sickened most seriously through excess of passion. Then were several physicians called to restore him, but, despite their careful watching, they could not guess his disease, and despaired of his recovery. And so it came to pass that one day, while a physician, still young indeed, but of profound science, sat near the patient, holding his arm there where the pulse beats, Giannetta came for some reason into the room where the youth was lying, who, when he saw the maiden, did not indeed betray his emotion either by word or gesture, but felt the ardour of passion increase in his heart, wherefore his pulse began to beat more violently than before. This the physician incontinently noticed and wondered, but remained still to see how long these pulsations would continue. When Giannetta left the room, the pulse became calmer. The physician, now deeming he had discovered the reason of the malady, caused the maiden to be called on the pretext of having a question to put to her, he still holding the patient by the wrist. Scarcely had she come into the room than the youth’s pulse beat more rapidly, slackening once more at her departure. Whereupon the physician, believing himself in possession of the truth, arose, and taking the parents of the young man aside, said to them: “The health of your son needs not the physician’s art; it lies in Giannetta’s hands.”'