802 THE AMERICAN JOURNAL OF SOCIOLOGY
of our experience, of our science and philosophy. They hold the zones of projection under their control, exercising on them an inhibitory or stimulating action from the point of view of our acts. To them alone belong conscious and especially methodical action.
The large posterior center of association plays perhaps the most important role. It is highly developed among intelligent men, but small among defectives. It may be noted that it is inserted between three of the most important sources of impres- sions: the tactile, auditory, and visual zones. On the contrary, the middle center, insula de Reil, is only between the tactile and olfactory sphere. The most scientific and precise impressions are then in relation with the former.
Thus in the constitution and evolution of the human nerve substance we have the substratum of intellectual development. In the prosencephalon the sensitive centripetal fibers are the first to develop the myelines and function. They appear at about the eighth month of the intra-uterine life. When these sensi- tive fibers have appeared, then the sensations stimulate the centers, and the myeline appears in the fibers of centrifugal pro- jection toward the end of the first month of the extra-uterine life. At this period only the centers of projection have devel- oped the myeline and become active; the acts of the child are still cortical reflexes ; the memory of impressions is not yet fixed in the brain. The latter from this point of view resembles animals without centers of association. All the impressions are received singly and are not compared. In the second month the myeline fibers appear and start from the sensorial spheres, going to the zones of association, and at first to the neighboring parts of the zones of projection. Then the memory of impres- sions is stored up. The child begins to recognize what it feels, sees, etc. Then the fibers of association, penetrating still farther, connect certain different spheres ; for example, the auditory sphere with the lower part of the tactile sphere ; the child is able to repeat the words heard, without comprehending their meaning. Finally, all the spheres of association being connected, the images can be compared. The child will reason, comprehend,
