CRANIAL NERVES. 



75 



this area is the beginning of the posterior white column (fig. 87) : the further course 

 and attachments of the ingrowing fibres in the cord are not accurately known, but 

 they appear to bifurcate and extend both upwards and downwards (Ramon y Cajal). 

 The peripherally directed fibres grow downwards and join the bundle of fibres of the 

 anterior root, to form together with them the mixed spinal nerve (fig. 88). 



The axis-cylinder processes which are to form the fibres of the anterior roots 



Fig. 86. BIPOLAR CELL FROM SPINAL GANGLION OP A 4| WEEKS EMBRYO. (His.) n f 



Fig. 87. SECTION OF SPINAL CORD OF FOUR WEEKS HUMAN EMBRYO. (His.) 



The posterior roots are continued within the cord into a small longitudinal bundle which is the 

 rudiment of the posterior white column. The anterior roots are formed by the convergence of the 

 processes of the neuroblasts. The latter, along with the elongated cells of the myelospongium compose 

 the grey matter. The external layer of the cord is traversed by radiating fibres which are the outer ends 

 of the spongioblasts. The anterior commissure is beginning to appear. 



begin to make their appearance about the beginning of the fourth week in the human 

 embryo (His). Their growth towards the periphery is glow; even by the end 

 of the second month they have not reached the tips of the fingers and toes. 



Cranial nerves. The neural crest is continuous along the dorsal aspect of the 

 cerebral part of the neural tube as far as, and even beyond the mid-brain. As in 

 the spinal part, clavate enlargements occur here also, but at somewhat irregular 

 intervals, and form ganglion rudiments which become separated from the dorsal aspect 

 of the tube, and acquire a new attachment on the lateral and ventral aspect. Such 

 ganglion rudiments have been described for the third, fifth, seventh, eighth, ninth, 

 and tenth nerves. 



In the chick the ganglion rudiments belonging to the cranial nerves appear as a 

 thickening of the cephalic epiblast, just where it is folding round into the as yet 

 unclosed neural canal (fig. 89, vg). This thickening, according to Golowine, is 

 continuous laterally with a modified portion of the external epiblast (sensory 

 epiblast), and soon becomes subdivided into three gangiionic groups, and these, later, 

 into separate ganglions. There is a corresponding subdivision of the sensory 



