THE SPINAL CORD. 209 



within a complex sponge-like matrix formed by the feltwork of various proc- 

 esses dendrites, axones and collaterals from mostly other neurones, the 

 supporting neurogliar reticulum and the blood-vessels. In two localities, 

 immediately around the central canal and capping the dorsal cornua, the 

 gray matter varies in appearance and constitution and exhibits the modifica- 

 tions peculiar to the central and Rolandic substantia gelatinosa. 



The nerve-cells of the anterior cornu are multipolar, the cell-bodies 

 appearing irregularly polygonal in cross-sections and fusiform in longitudinal 

 ones, and measure from 65-135 fi in diameter unless unusually small, when 

 their diameter may vary from 3080 /JL. The most conspicuous and important 

 elements of this region of the gray matter are the motor radicular cells, 



FIG. 315. Isolated root -cell from anterior horn of cord of calf ; a, axone. X 210. 



whose axones pass from the apex of the cornu, traverse the white matter 

 (meanwhile becoming medulla ted), and emerge from the cord as the axis- 

 cylinders of the efferent (motor) root-fibres of the spinal nerves. These 

 cells possess from three to ten dendritic processes, which radiate in various 

 planes, divide dichotomously and finally end in terminal arborizations that 

 may reach the posterior horn and other parts of the gray matter. In contrast 

 to the robust dendrites beset with spines, the axone is slender smooth and 

 directly continuous with a root-fibre of a spinal nerve. With the exception 

 of delicate collaterals, which may be wanting, the axone is unbranched. 

 Each nerve-cell possesses a spherical or ellipsoidal nucleus ( 10-20 /.*) , enclosed 

 by a distinct nuclear membrane, and usually a single nucleolus, exceptionally 

 more than one. In addition to accumulations of deeply staining tigroid sub- 

 stance, the cytoplasm contains brownish-yellow pigment granules, often in the 

 vicinity of the implantation cone from which the axone springs. 



In addition to the foregoing radicular cells, the anterior cornu contains other 

 nervous elements, the commissural and strand-cells. The commissural cells occur 

 chiefly within the median part of the anterior horn and resemble in size and form the 

 radicular cells, but possess smaller nuclei. Their axones traverse the anterior white 

 commissure to gain the opposite anterior column, in which they divide T-like into 

 ascending and descending fibres, or undivided turn brainwards. The strand-cells, 

 variable in form and generally smaller than the root-cells, are only sparingly repre- 



