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ELEMENTS OF MAMMALIAN ANATOMY 



The elements of the spinal cord, like those of the brain, are 

 of two kinds — the cells composing the gray matter and the 

 fibers composing the white matter. While in the brain the 

 gray matter is largely on the surface, forming the cortex, in 

 the cord it occupies the central region. A canal, the canalis 

 cciilralis, about one-half a millimeter in diameter, extends 

 throughout the cord, opening into the fourth ventricle of the 



Fig. 112. — Cell from the Central Nervous System. X loo. 

 a, Dendrites or protoplasmic processes; ax, axone; n, nucleus of the cell body. 



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 ventral horns, and the posterior columns, 

 the dorsal horns (Fig. in). 



Many of the fibers extend in a longitudinal direction through- 

 out the cord, but the roots of the spinal nerves upon entering 

 the cord run transversely a longer or shorter distance, and in 

 many cases cross to the opposite side. Numerous experiments 



