196 ELEMENTS OF MAMMALIAN ANATOMY. 
the canalis centralis, about one-half a millimeter in diameter, 
extends throughout the cord, opening into the fourth ven- 
tricle of the brain. A cross-section of the cord shows the 
gray matter arranged in the shape of a letter H. The 
ventral columns of gray matter are the anterior horns, and 
the posterior columns, the posterior horns (Fig. 100). 
Many of the fibers extend in a longitudinal direction 
throughout the cord, but the roots of the spinal nerves upon 
entering the cord run transversely a longer or shorter dis- 
Fic. tor. CrLt FRoM THE CENTRAL Nervous SysTEM. %X 100. 
a, Dendrites or protoplasmic processes; ax, axone; n, nucleus of the 
cell body. 
tance, and in many cases cross to the opposite side. Num- 
erous experiments show that the anterior root fibers are, for 
the most part, the axones of the cells in the anterior horn 
(Fig. 102). 
