THE SPINAL COED AND SPINAL NEEVES. 



525 



The ventral or anterior commissure of the spinal cord. The 



neuroblasts of the dorso-lateral areas of the cord also give off 

 nerve processes ; but these, in place of passing out beyond the 

 cord, run in its walls. Some of the nerve fibres take a longi- 

 tudinal course, and give rise to the white columns of the cord ; 

 while others (Fig. 226) run downwards to its ventral surface, 

 interlacing with the fibres of the motor roots, and, on reaching 

 the mid-ventral surface, pass across to the opposite side of the 

 cord, and so give rise to the ventral or anterior commissure. 



Fig. 226. — A diagrammatic transverse section across the spinal cord of a 

 Human Embryo of tile fourth week. (After His.) x 150. 



UC, central canal of spinal cord. UD, dorsal root of spinal iiorve, T3"I, nuclei of 

 spongioblasts. WV, ventral or motor roots of spinal nerve. !N~W", ventral columns 

 of white matter. NZ, neuroblast. 



The dorsal or sensory nerve roots. The early origin of the 

 spinal ganglia in the human embryo has not been made out 

 very satisfactorily; so far as is known, it agrees in all essential 

 respects with that already described as occurring in chick 

 embryos. 



In Kollmann's embryo, estimated as fourteen days old 

 (Fig. 185), the ganglion rudiments are described by Lenhossek 

 as arising before closure of the neural canal is effected, appear- 

 ing in transverse sections as small heaps of rounded cells, the 



