THE NEOPALLIUM. 



345 



areas give by far the most simple and direct and probably the 

 most accurate means of mapping them out. The clinical and 

 pathological data come into account here, but the most illuminat- 

 ing studies are those based upon the order of myelinization of 

 the fiber tracts (method of Flechsig). Flechsig has shown that 

 in the developing human brain certain fiber tracts produce their 

 myelin sheaths earlier, others later, and that the order of mye- 

 linization is measurably regular and constant. Certain practical 

 advantages in tracing fiber tracts were gained by this discovery. 

 The tracts which are myelinated early may often be traced with 

 great ease in early stages before adjacent tracts have received 

 their myelin. When the tracts which are myelinated early are 

 fully known in early stages of development the tracts which become 

 myelinated later may be studied with greater certainty. The 

 great advantage for the study of cerebral localization, however, 

 lies in the fact that the majority of fibers connected with any nerve 

 center or engaged in carrying impulses of the same category, 

 become myelinated at the same time. If, then, the fibers which 

 go up to the cortex from the centers for hearing, for example, 

 become myelinated at the same time, the area of the cortex con- 

 cerned with hearing will be indicated by the distribution of this tract 

 when it is first myelinated and can be traced most easily. Further- 

 more, the production of myelin seems to be the result of the begin- 

 ning of functional activity of nerve fibers and the" myelin appears 

 first near the cells of origin and is progressively formed toward the 

 end of the fiber and so indicates the direction in which the impulses 

 travel. Thus, when a sufficient series of developing brains are 

 studied this method should give three classes of facts: first, the 

 course of the fibers engaged in a specific function and the cortical 

 area to which they are related; second, the order in time when 

 various nervous pathways begin to function; and third, the direc- 

 tion in which impulses are carried by given fiber tracts. 



THE SENSO-MOTOR AREAS. The afferent or sensory fibers 

 which come to the neopallium from lower brain centers end in 

 certain regions, and from these regions alone (Flechsig) arise the 

 great majority of the efferent or motor fibers which carry impulses 

 to the motor centers of the brain and spinal cord. Hence these 



