THE NERVOUS SYSTEM 241 
and descending tracts of fibers derived from medullary 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 
Fic. 144. — Transverse section through the cervical swelling 
of the spinal cord of a chick, middle of the fifth day. (After 
v. Kupffer.) 
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 
