THE SPINAL CORD. 



271 



however, they may be divided into: (i) the cells of Clarke 1 s column, (2) 

 the cells of the substantia gelatinosa Rolandi, and (3) the inner cells of the 

 caput cornu, 



The cells of Clarke's column form a conspicuous collection that occupies the 

 mesial border of the base of the posterior horn (Fig. 317), and correspond to an 

 elevation on the surface of the gray matter. The cells are about 50 / in diameter, 

 polygonal in outline and possess many richly branched dendrites that radiate chiefly 

 within the limits of the group. The axones course ventrally for some distance before 

 bending outward toward the lateral column within which, as the constituent fibres of 

 the direct cerebellar tract (fasciculus cerebello-spinalis), they turn brainwards. 



The nerve-cells of the substantia gelatinosa include innumerable small stellate 

 elements, less frequently fusiform or pear-shaped, that measure only from 6-20 //, 

 although exceptionally larger. Their numerous short dendrites are irregularly 



White matter o 

 posterior columr 



Cells of Clarke's column 



Substantia gelatinosa centralis 



Central canal 

 FIG. 317. Part of cord, showing cells of Clarke's column in base of posterior horn. X no. 



disposed and branched. The axones are continued partly into the adjacent white 

 matter of the posterior column, where they divide into ascending and descending 

 limbs, and partly into the gray matter, in which they run as longitudinal fibres. The 

 marginal cells are fusiform or pyramidal and larger (35-55 y") and occupy the border 

 of the substantia gelatinosa. Their axones traverse the substantia gelatinosa and 

 probably continue, for the most part, within the lateral column, although some enter 

 the posterior. Many of the nerve-cells within the substantia gelatinosa were formerly 

 regarded as glia cells, an exceptionally large amount of neuroglia being credited to 

 this substance; instead of such being the case, this region of the gray matter is 

 relatively poor in neuroglia and numerically rich in nerve-cells. 



The inner cells of the posterior cornu, intermingled with numerous nervous 

 elements of small size, are triangular or spindle-shaped and usually measure about 

 50 n\ they are, therefore, larger than the ordinary cells of the substantia gelatinosa. 

 Their axones pass mostly into the lateral column of the same side; some, however, 

 have been traced into the posterior or anterior columns of the same and, rarely, into 



