412 THE CENTRAL NERVOUS SYSTEM. 



sory neurones are situated in the spinal ganglia found on the pos- 

 terior roots of the spinal nerves. In the embryo they are distinctly 

 bipolar, but during further development their two processes approach 

 each other, and then fuse for a certain distance, forming finally 

 single processes which branch like the letter T. In reality, then, 

 there are two processes which are fused for a certain distance from 

 the cell-body of each neurone. The peripherally directed process 

 is regarded as the dendrite of the cell, and the proximal as the 

 neuraxis passing to the spinal cord. The neuraxes enter the spinal 

 cord through the posterior roots and pass to the posterior columns, 

 where they divide, Y-shaped, into ascending and much shorter 

 descending branches, from each of which numerous collateral 

 branches are given off. 



From the preceding account of the white matter of the spinal 

 cord, it may be seen that it consists of longitudinally directed neu- 

 raxes arranged in so-called short and long tracts or columns. The 

 neuraxes constituting the former, after a short course through the 

 gray matter, emerge from it, and after giving off various collaterals, 

 again penetrate into the gray matter, where their telodendria enter 

 into contact with the ganglion cells. The long columns consist of 

 the neuraxes of neurones the cell -bodies of which are situated in 

 the cerebrum or cerebellum, and of neurones the cell-bodies of which 

 are in the spinal cord or spinal ganglia and the neuraxes of which 

 terminate in the medulla or cerebellum. The nerve-fibers of the 

 various columns give off numerous collaterals which enter the gray 

 matter to end in telodendria. The collaterals of the posterior col- 

 umns end : (i) between the cells of the gelatinous substance of the 

 posterior horns ; (2) in the columns of Clark ; (3) in the anterior 

 horns, these constituting the principal portion of the so-called reflex 

 bundles ; (4) in the posterior horn of the opposite side. The col- 

 laterals of the lateral columns pass horizontally toward the central 

 canal, some ending in the anterior horn, others closely arranged 

 near the columns of Clark, and some arching around the central 

 canal, forming with the corresponding fibers of the other side the 

 anterior bundles of the posterior commissure. The collaterals of 

 the anterior columns form well-marked plexuses in the anterior 

 horns of the same and opposite sides. 



We have still to describe the two commissures. The anterior 

 consists of: first, neuraxes from the commissural cells ; second, 

 dendrites from the lateral group of the anterior horn cells ; and, 

 third, the collaterals of the anterolateral column, which end in the 

 gray substance of the other side of the cord. The posterior com- 

 missure is probably composed of the collaterals from all the remain- 

 ing columns. The posterior bundle of this commissure comes 

 from the posterior column ; the middle, from the posterior portion 

 of the lateral column ; and the anterior, or least developed, from 

 the anterior portion of the lateral column, possibly also from the 

 anterior column. 



