168 THE EIDDLE OF THE UNIVERSE. 



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-layei\ 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 psycho- 

 logy 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. 



The long dorsal cylindrical medullary tube which 

 is thus formed is thoroughly characteristic of the 



