THE NERVOUS SYSTEM. 



207 



matter are the anterior horns, and the posterior columns, 

 the posterior horns (Fig. 96). 



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 distance, and in many cases cross to the opposite 

 side. It is thought that the anterior root fibers are, for 



FIG. 96. DIAGRAMMATIC CROSS-SECTION OF THE SPINAL CORD. The cells 

 and fibers are represented too few and too large in proportion to the 

 size of the cord. 



ag, Anterior horn or column of gray matter; a, nerve process of the cell 

 c; an, anterior root ; cc, canalis centralis ; c, one of many cells forming 

 the ganglion on the posterior root; d, posterior fissure; e, cell giving 

 off an axis-cylinder process into the anterior root ; dn, dorsal branch ; 

 /, cut-off fibers; gn, ganglion of posterior root; i, axis-cylinder pro- 

 cess of the cell io; nc, junction of anterior and posterior roots; ps, 

 posterior root of nerve; pg, posterior horn of gray matter; s, terminal 

 arborization of part of fiber from c , v, anterior median fissure; in, 

 ventral branch of nerve; van, axis-cylinder process of anterior horn 

 cells ; x, the part of the posterior root fiber extending caudad in the 

 cord. 



the most part, the axis-cylinder processes of the cells in 

 the anterior horn (Fig. 98). 



Each posterior root fiber after passing into the cord 

 separates into two parts one of which extends cephalad, 

 the other caudad (Fig. 98). Both give off branches at 

 right angles, called collaterals, which terminate in arbori- 

 zations about the cells of the cord. 



