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- 



FIG. 101. CELL 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). 



