Each of the higher animal species has a characteristic psychic organ; the central nervous system of each has certain peculiarities of shape, position, and composition. The medusæ, among the radiating cnidaria, have a ring of nervous matter at the border of the fringe, generally provided with four or eight ganglia. The mouth of the five-rayed cnidarion is girt with a nerve-ring, from which proceed five branches. The bi-symmetrical _platodes_ and the _vermalia_ have a vertical brain, or acroganglion, composed of two dorsal ganglia, lying above the mouth; from these "upper ganglia" two branch nerves proceed to the skin and the muscles. In some of the vermalia and in the mollusca a pair of ventral "lower ganglia" are added, which are connected with the former by a ring round the gullet. This ring is found also in the _articulata_; but in these it is continued on the belly side of the long body as a ventral medulla, a double fibre like a rope-ladder, which expands into a double ganglion in each member. The vertebrates have an entirely different formation of the psychic organ; they have always a spinal medulla developed at the back of the body; and from an expansion of its fore part there arises subsequently the characteristic vesicular brain.[20] Although the psychic organs of the higher species of animals differ very materially in position, form, and composition, nevertheless comparative anatomy is in a position to prove a common origin for most of them--namely, from the vertical brain of the platodes and vermalia; they have all, moreover, had their origin in the outermost layer of the embryo, the _ectoderm_, or outer skin-layer. Hence we find the same typical structure in all varieties of the central nervous organ--a combination of ganglionic cells, or "psychic cells" (the real active elementary organs of the soul), and of nerve-fibres, which effect the connection and transmission of the action. The first fact we meet in the comparative psychology of the vertebrates, and which should be the empirical starting-point of all scientific human psychology, is the characteristic structure of the central nervous system. This central psychic organ has a particular position, shape, and texture in the vertebrate as it has in all the higher species. In every case we find a spinal medulla, a strong cylindrical nervous cord, which runs down the middle of the back, in the upper part of the vertebral column (or the cord which represents it). In every case a number of nerves branch off from this medulla in regular division, one pair to each segment or vertebra. In every case this medullary cord arises in the same way in the foetus; a fine groove appears in the middle axis of the skin at the back; then the parallel borders of this medullary groove are lifted up a little, bend over towards each other, and form into a kind of tube.