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.