THE XERVOUS SYSTEM 



241 



and descending tracts of fibers derived from medullarv and 

 ganglionic neuroblasts. The dorsal and ventral roots of the 

 spinal nerves divide it on each side into three main columns. 

 viz., dorsal situated above the dorsal root, lateral situated be- 

 tween dorsal and ventral roots, and ventral situated below the 



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Fig. 144. — Transverse section through the cervical swelhng 

 of the spinal cord of a chick, middle of the fifth day. (After 

 V. Kiipffer.) 



bl. v., Blood vessel. C. a., Anterior commissure. C, Cen- 

 tral canal, d., Group of axones at the level of the dorsal root. 

 Ep., Ependyma. N'bl., Neuroblasts. V. Ventral column of 

 white matter. 



ventral roots. The dorsal column begins first as a bundle of 

 fibers at the entrance of the fibers of the dorsal root (Fig. 144). 

 Subsequently, other fibers come in this region and gradually 

 extend towards the dorsal middle line, displacing the ependyma 



