The famous pineal gland has lately been recognized by comparative anatomists as the rudiment of a single organ of vision, the pineal eye (which is still found in certain reptiles). Moreover, not one of the innumerable psychologists who seek the seat of the soul in some part of the body, after the fashion of Plato, has yet formulated a plausible theory of the connection of mind and body and the nature of their reciprocal action. On our monistic principles the answer to this question is very simple, and consonant with experience. In view of its extreme importance, it is advisable to devote at least a few lines to the consideration of the phronema in the light of anatomy, physiology, ontogeny, and phylogeny. When we conceive the phronema as the real "organ of the soul" in the strict sense--that is to say, as the central instrument of thought, knowledge, reason, and consciousness--we may at once lay down the principle that there is an anatomical unity of organ corresponding to the physiological and generally admitted unity of thought and consciousness. As we assign to this phronema a most elaborate anatomical structure, we may call it the organic apparatus of the soul, in the same sense in which we conceive the eye as a purposively arranged apparatus of vision. It is true that we have as yet only made a beginning of the finer anatomic analysis of the phronema, and are not yet able to mark off its field decisively from the neighboring spheres of sense and motion. With the most improved means of modern histology, the most perfect microscopes and coloring methods, we are only just beginning to penetrate into the marvellous structure of the phronetal cells and their complicated grouping. Yet we have advanced far enough to regard it as the most perfect piece of cell-machinery and the highest product of organic evolution. Millions of highly differentiated phronetal cells form the several stations of this telegraphic system, and thousands of millions of the finest nerve-fibrils represent the wires which connect the stations with one another and with the sense-centres on the one hand, and with the motor-centres on the other. Comparative anatomy, moreover, acquaints us with the long and gradual development which the phronema has undergone within the higher class of the vertebrates, from the amphibia and reptiles up to the birds and mammals, and, within the last class, from the monotremes and marsupials up to the apes and men. The human brain seems to us to-day to be the greatest marvel that plasm, or the "living substance," has produced in the course of millions of years.