THE BRAIN AND SPINAL CORD. 197 



the fibres of the posterior roots. They become myelinated 

 out the 6 th month. 



(2) To the inner side and to the outside of the anterior horn 

 ires are produced from cells in the anterior horn, which con- 

 ct together neighbouring spinal segments (association fibres). 



(3) Two tracts of fibres grow out from cells in the grey matter 

 the cord and go to the cerebellum. They are the direct cere- 

 liar tract (Flechsig's) from the cells of Clarke's column and 

 Dwer's tract from cells at the base of the anterior horn. 



(4) Fibres produced by the cells of the cerebellar cortex grow 

 iwnwards outside the anterior horn (intermediate tract). All 

 ese tracts are medullated before birth. 



(5) The pyramidal tracts (crossed and direct) grow down from 

 e cells of the motor cortex. They are not medullated until 

 r e months after birth. 



The white matter of the cord is thus entirely produced by the 

 growth of nerve fibres within the neuroglial network of the 

 iter zone. The cerebral motor cortex, through the pyramidal 

 acts, comes to dominate the cord ; sensory and cerebellar paths 

 e formed, and intercommunications set up between the spinal 

 gments. 



The Anterior and Posterior Median Fissures are produced by the 

 owth of the white matter. It is possible that the posterior 

 edian fissure is formed in part by the inclusion of the dorsal 

 gment of the central canal of the neural tube. 



B. The Middle Zone — filled with neuro-blasts, forms the horns 

 id grey matter of the cord. 



C. The Inner Zone becomes the ependyma which lines the 

 ntral canal and the gelatinous tissue which surrounds it. 

 ie columnar cells which line the central canal are ciliated. The 

 ntral canal retains the embryonic calibre while the wall 

 creases enormously in thickness. 



THE HIND BRAIN. 



That part of the neural tube which forms the hind brain 

 ig. 159) becomes transformed into: 

 (1) The Medulla Oblongata. 



