4O8 THE CENTRAL NERVOUS SYSTEM. 



sures and the central canal of the spinal cord, while the vertical 

 limbs or crescents extend to the ventral and dorsal nerve-roots, 

 forming the anterior and posterior horns. The former are, as a 

 rule, the larger, and at their sides (laterally) the so-called lateral 

 horns may be seen, varying in size in different regions. In each 

 anterior horn are three main groups of ganglion cells : the ventro- 

 lateral, made up of root or motor nerve-cells ; the ventromesial, 

 composed of commissural cells ; and the lateral (in the lateral 

 horn), containing column cells. At the median side of the base 

 of each posterior horn we find a group of cells and fibers known 

 as the column of Clark, most clearly defined in the dorsal region, 

 while in the posterior horn itself is the gelatinous substance of 

 Rolando. Aside from these, numerous cells and fibers are scat- 

 tered throughout the entire gray substance. 



The motor nerve-cells lie in the ventrolateral portion of the ante- 

 rior horn, their neuraxes extending into the anterior nerve-root. 

 Their dendrites are distributed in a lateral, dorsal, and mesial direc- 

 tion, the two former groups ending in the anterior and lateral col- 

 umns, the mesial in the region of the anterior commissure. Some 

 of the mesial dendrites extend beyond the median line and form a 

 sort of commissure with the corresponding processes of the other 

 side. The commissural cells lie principally in the mesial group of 

 the anterior horn, but occur here and there in other portions of the 

 gray substance. Their neuraxes form the anterior gray commis- 

 sure with the corresponding processes from the other side. After 

 entering the white substance of the other side, these neuraxes 

 undergo a T-shaped division, one branch passing upward and the 

 other downward. The column cells are small multipolar elements, 

 represented by the cells of the lateral horns, although they are 

 also found throughout the entire gray mass. Their neuraxes pass 

 directly into the anterior, lateral, and posterior horns. 



The cells of the column of Clark, or nucleus dorsalis, are of two 

 kinds those in which the neuraxes pass to the anterior commis- 

 sure (commissural cells) and those in which the neuraxes pass into 

 the direct cerebellar tract of the same side. The plurifunicular 

 cells are cells the neuraxes of which divide two or three times in 

 the gray substance, the branches then passing to different columns 

 of the white matter on the same or opposite side of the cord. In 

 the latter case the branches must necessarily extend through the 

 commissure. The cells of the substantia gelatinosa (Rolando) are 

 cells with short, freely branching neuraxes, which end after a short 

 course in the gray mass (Golgi's cells). The posterior horn con- 

 tains marginal cells, spindle-shaped cells, and stellate cells. The 

 first are situated superficially near the extremity of the posterior 

 horn, their neuraxes extending for some distance through the gela- 

 tinous substance of Rolando and then into the lateral column. The 

 spindle-shaped cells are the smallest in the spinal cord and possess a 

 rich arborization of dendrites extending to the nerve -root of the pos- 



