CH. L.] FUNCTION AND MYELINATION 739 



functions which are so important in the superintendence of other 

 people. Mott's observations on lunatics show that this region is 

 important for intellectual operations, though not so important as the 

 parietal association area behind the Eolandic area; the greater the 

 intellectual development, the larger and more convoluted does this 

 parietal region become. 



The association fibres have been the subject of special study by 

 Flechsig, who has shown that in the development of the brain these 

 are the last to become myelinated ; white fibres do not become fully 

 functional until they receive their medullary sheath. This coincides 

 with the well-known fact that association of ideas is the last phase in 

 the psychical development of the child. It has been shown that the 

 frontal convolutions are connected by important association tracts 

 with the more posterior regions of the brain (see fig. 436, p. 699), 

 and there is therefore no difficulty in understanding that the frontal 

 convolutions play the part of a centre for the association of ideas, or 

 in other words for intellectual operations. 



Function and Myelination. 



Flechsig's embryological method has given us most valuable knowledge of the 

 structure and functions of the human brain. The method depends on the fact that 

 various tracts of fibres become myelinated, i.e. 9 acquire their medullary sheath at 

 successive periods of time in development. The myelin sheath appears three or four 

 months after the axis cylinder is formed. The Weigert method of staining renders 



c.c 



M.O: 



Fin. 452. Diagram of vertical section through brain of new-born child, drawn from one of Flechsig's 

 photographs. The section was treated by Weigert's method, by which myelinated fibres are deeply 

 stained. Attention is drawn to the deep shading indicating myelination around the central fissure, 

 which corresponds to the sensori-motor area, and also around the calcarine fissure in the visual 

 sphere. The association fibres are not myelinated. The fibres of the pyramidal efferent system 

 have also no myelin. M.O., medulla oblongata ; P.V., pons Varolii ; O.M.N., oculo-motor nerve; 

 O.C., optic commissure; F.A.C., frontal association centre; C.C., corpus callosum ; C. P., central 

 fissure, or fissure of Rolando; P.A.C., posterior association centre; V.S., visual sphere; C., cere- 

 bellum ; S.C., spinal cord. 



the detection of a medullary sheath an easy task. Flechsig's method is in short the 

 complement of the Wallerian method. In the former method the tracts are isolated 

 by the differences in the origin of the myelin sheath ; in the latter method, the same 

 object is obtained by observing the degeneration which is most noticeable in the 

 same sheath. 



In the central nervous system, the afferent projection fibres are myelinated first ; 



