THE CORTEX AND MEDULLARY CENTER OF THE CEREBRAL HEMISPHERE 289 



Myelination. The fibers in the various parts of the cortex acquire their 

 myelin sheaths at different tunes. On this basis Flechsig (1896) identified 

 thirty-six areas, which are numbered in Fig. 219 in the order of myelination. He 

 recognizes three main groups: primary (Nos. 1 to 12), intermediate (Nos. 13 



Fig. 218. Diagram showing the differences in thickness and in the arrangement of the lighter 

 and darker bands in the human cerebral cortex in different regions as seen with the naked eye: 

 A, Motor cortex from anterior central gyrus; B, sensory cortex from the posterior central gyms; 

 C, visual cortex from the region of the calcarine fissure; D, auditory cortex from the anterior 

 transverse temporal gyrus. (Redrawn after Elliot Smith.) 



to 28), and late (Nos. 28 to 36). According to Flechsig, the primary areas, 

 which are myelinated at birth, are projection centers and receive the sensory 

 radiation from the thalamus; while the other parts of the cortex, not being pro- 

 vided with projection fibers, serve only as association centers. He believed that 



Fig. 219. Lateral view of the human cerebral hemisphere, showing the cortical areas as 

 outlined by Flechsig on the basis of differences in the time of myelination of their nerve-fibers. 

 The primary areas (first to become well myelinated) are cross-hatched; the intermediate are 

 indicated by vertical lines; the late areas are unshaded. (Lewandowsky.) 



myelination of nerve-fibers takes place in the order of conduction, that is, the 

 sheaths are developed first on the afferent fibers, reaching the cortex from the 

 thalamus, and later on the association fibers, linking the various areas together. 

 According to this conception fibers of like function tend to become myelinated 

 19 



