692 FUNCTIONS OF THE CEREBRUM [CH. XLVIII. 



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., 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 

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

 reverse 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 ; 

 the efferent projection fibres and the association fibres are myelinated later. Thus 



C.F. 



C.C. 



F.A.C. 



FIG. 501. 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.F., central 

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

 bellum ; S.C., spinal cord. 



in the human foetus the peripheral nerves and nerve-roots become myelinated in the 

 fifth month of intra-uterine life ; of the tracts in the cord, those of Burdach and Goll 

 (exogenous fibres springing from the cells of the spinal ganglia) are the first to be 

 myelinated ; next come the tracts of Flechsig (dorsal cerebellar) and of Gowers 

 (ventral cerebellar) : these are endogenous fibres springing from cells within the cord. 

 All these tracts are afferent. The pyramidal tracts, the great efferent or motor 

 channels, are not myelinated until after birth. The whole afferent tract is myelinated 

 at birth ; these fibres have in utero been exercised in conveying impressions to the 

 afferent reception centres, the stimuli arising from contact of the foetal integuments 

 with the maternal tissues. There is also early myelination around the calcarine 

 fissure in the visual sphere, and in connection with the areas related to other special 

 senses. This is shown in figs. 501 and 502, where the condition at birth and that 

 some months later are compared. 



Flechsig considers that at least two-thirds of the cortex consists of neurons of 

 association, and further that these association centres possess no neurons of the 

 efferent or afferent projection systems. The last part of this statement is prob- 

 ably not correct, and has not been accepted in its entirety by the majority of 

 neurologists. 



Ambronn and Held confirm Flechsig in finding that the afferent fibres are 



