In Plato's estimation the mortal soul, whose chief characteristic is the virtue of " fortitude and spirit and which loves contention," is located in the thorax. Thorax in this connection stands for that part of the body which begins with the collar-bone and reaches as far down as the pelvis. Here " they (the gods) placed the heart in a kind of sentry-house." The Appetites are like wild beasts chained to a manger, which Reason, dwelling in the head, controls by the aid of the passions, which, in turn, to a large extent, are held in check by the heart. Thus every passion, no matter in what part of the soul it originates, passes through the heart. Here may be found the explanation of the popular concept of the function of that organ. Plato, however, is not the first who attributed such importance to the heart in relation to passions. A similar belief was expressed in Jewish literature and the Sacred Scriptures long before his time.
(C) The Platonic concept of the heart implies a participation of this organ in man's moral life. Such a function presupposes an ascendancy exercised by it over the appetitive faculties. In conformity with this view the physiologists of the past accepted the current belief of the common people, and thus established the mistaken conclusion that the heart is the seat of love. The Scholastics, accepting this view, pictured the heart as the medium or channel, in which resided (in a dormant state, so to say) man's emotional, intellectual, as well as other operations, which, in order to be animated to action, needed only the stimulus of need, environment or other circumstances.
Such, however, is not the present-day conception of the function which the heart is called upon to perform in the human organism. Lewes developes what might be called the modern physiological theory of the heart. The experimentalists, to all appearances, have proved conclusively that the functions formerly attributed to the heart are in reality exercised by the brain. Even if this theory be not open to question, we need not be disconcerted. The interrelation existing between the heart and the brain will still afford a sufficient ground for maintaining the symbolism involved in the Devotion to the Sacred Heart.
No physiologist would deny the importance of the pulmonary circulation. The sensibility of the human body depends to a great extent on the blood. Even the extra-vascular tissues of the body are dependent on it. The right side of the heart receives the blood as it flows in from the general system of the veins, and sends it on to the lungs. The left side receives it from the lungs and sends it into the general system. Thus the blood exercises an enormous influence upon the vegetative phenomena, upon the life and, to a great extent, upon the individual dynamic activity of the nerve-cells; consequently, the life of the whole system depends upon it. The continuity of the irrigation of the blood is, then, a conditio sine qua non of the regular working of the cerebral cells. According to the estimate of Haller, one-fifth of the whole blood-supply goes to the brain.
But in the human organism the cardiac and the cerebral systems so interlace and interpenetrate each other that the slightest modification of one is followed by a corresponding change in the other. The brain is nourished by the heart, and the latter's blood, momently pumped into it, makes the activity of the former possible- The transmission of the blood to the lungs and its distribution throughout the organism are rendered possible only through the influence of the brain. The brain of all the organs is the most delicately susceptible to every variation in the quality of blood sent to it. If the heart pumps feebly, the brain acts feebly. If the blood is vitiated, the brain becomes lethargic. Thus these two great centres interact. They are both lords of life and mutually indispensable.
On the other hand, every impression reacts on the circulation, a slight impression producing a slight acceleration, a powerful impression producing an arrest more or less prolonged. We are all familiar with the fact that powerful mental shocks arrest the action of the heart. Sometimes this arrest is final, as in the case of a very sensitive and sickly organism. The degree of sensitiveness depends on the connection that exists between the heart and the brain. "Thus the heart is indissolubly connected with every manifestation of sensibility; and it is so delicately susceptible to all emotional agitation, that we may not improperly regard it, as the ancients regarded it, in the light of the chief centre of feeling; for the ancients had no conception of the heart as an organ specially endowed with sensibility, they thought of it only as the chief agent of the sensitive soul." " Hence the heart, because its action is momently involved in every motion, and because every emotion reacts upon it, may, as truly as the brain, be called the great emotional centre." Neither brain nor heart can claim that title exclusively, but the two may claim it together.