The heart continues to beat in an orderly and regular manner even when quite removed from the body, and in the process of development the speck which represents it in the young begins to beat rhythmically long before any nerves are formed. These circumstances show that its action is to a large extent independent of the great centres of the nervous system, whilst on the other hand the readiness with which the heart responds to disturbing conditions of the general system, in regard alike to the frequency and the strength of its beats, clearly indicates that it is under the control of certain nerves which can be demonstrated by anatomical as well as by physiological evidence to have intimate relations with it. The nerves distributed to the heart are derived from the spinal cord and medulla oblongata, as well as from the sympathetic nerve. They have been divided into the accelerating and the restraining or inhibitory nerves. The accelerating nerves leave the spinal cord at the fore part of the dorsal region, and pass to the first dorsal ganglion of the great sympathetic chain, and after a short course are distributed to the heart. Stimulation of these branches causes the heart to beat more frequently.
The inhibitory or restraining nerves of the heart are derived from the medulla oblongata, and run in the vagus nerve. Their peculiarity is that, instead of causing contraction of the muscle to which they are distributed, they induce relaxation, so that when stimulated they stop the heart in diastole, that is, in a condition of relaxation; whilst when less strongly stimulated they cause it to beat more slowly. The effect is not, however, persistent, for even if the vagal branches continue to be strongly stimulated, the heart after a time recommences to beat more quickly and more strongly than before. The administration of belladonna, or of its active principle atropine, prevents the inhibitory effect from manifesting itself when the vagus is stimulated, and a similar action is exerted by curara and by nicotine. On the other hand, muscarin. a poison obtained from a mushroom (Amanita muscaria), seems to stimulate or intensify the inhibitory influence.
The heart is in unceasing movement day and night. Yet it has, like other muscles, its period of rest; for expenditure of force is only taking place during contraction, which occupies about one-half of the whole cycle of its action. It differs, therefore, from the ordinary muscles that are under the control of the will only in the circumstance that, instead of long spells of greater or less activity occurring alternately with the complete rest of sleep, its periods of work and rest have only short intermissions. The force it exerts is immense. If we estimate that the quantity of blood driven out of the left ventricle at each contraction is the low amount of 1 pint or pound, and that it is raised about 10 feet, which represents the blood pressure, the work done is 10 foot-pounds per beat. Taking the _ number of beats at 40 per minute, we have 400 lbs. raised 1 foot per minute, or 1 lb. raised 400 feet. If this be multiplied by the number of minutes in an hour, and of hours in the day, the surprising number of 576,000 foot-pounds, or more than 257 foot-tons, raised in twenty-four hours is obtained, which represents the work done by the left ventricle. The work of the right ventricle is estimated at one-third of this amount. The duration of one complete circulation of all the blood is about 27 seconds.