[Illustration: Fig. 2.--CORTEX OF HUMAN BRAIN ILLUSTRATING COMPLEXITY OF THE SYSTEMS AND PLEXUSES OF NERVE FIBERS (Combination of the methods of Weigert and Golgi--after Andriezen). _c, z.,_ clear zone free from nerve fibers; _M.P.,_ Exner's plexus in the molecular layer; _A. str.,_ ambiguous cell stratum; _Subm, P.,_ sub-molecular plexus; _Gt. P. P.,_ great pyramidal plexus; _Pol. P.,_ polymorphic plexus; _W.,_ white matter. (Barker.) ] {110} These explanations are as yet only theoretic, but theories have often helped students in science to make their thoughts more concrete and their investigations more practical. It would be a mistake to conclude that because some of the theories advanced are very plausible, we have, therefore, reached definite truth with regard to the mechanics of the brain that underlie suggestion and mental influence. Brain Complexity.--The most interesting feature of the discoveries in brain anatomy during the past generation, has been that the central nervous system is of even greater complexity than had been thought. Because of this, these new discoveries, instead of solving the biological mystery they subtend, or even helping very much to solve it, have made it still harder to understand just how we succeed in controlling and directing this immensely complex machine, of whose details we are utterly unconscious, yet which we learn to use with such discriminating nicety of adjustment and accomplishment. The discoveries of Golgi and of Ramon y Cajal show us that the brain consists of nerve cells with a number of ramifying fibers connecting each cell and each group of cells with other simple and compound elements of the brain, and sending down connecting fibers to every organ and every part of the body. Dr. Ford Robertson calculates that in an average human brain there are at least three billions of cells. Without knowing anything of their existence, much less anything of the infinite detail of their structure and mode of operation, we have learned to use these for many purposes. [Illustration: FIG. 3.--SMALL AND MEDIUM-SIZED PYRAMIDAL CELLS OF THE VISUAL CORTEX OF A CHILD TWENTY DAYS OLD. Section taken from the neighborhood of the calcarine fissure. A. plexiform layer; B, layer of the little pyramid; C, layer of the medium-sized pyramid; a, descending axis cylinders; b, ascending or centripetal collaterals; c, stems of the giant pyramidal cells. (Ramon y Cajal.) (This and the next three illustrations illustrate the complexity of the central nervous system as observed in the very young child where the development does not as yet obscure the interesting details of dentritic branching. They serve to emphasize the much more pronounced condition which develops in the adult.)]