THE GRAY SUBSTANCE.
Exclusive of the supporting tissue, the gray substance consists principally of nerve cells with their protoplasmic and nerve-processes and nerve-fibres ending around the nerve-cells. Topographically regarded, four different cell-groups may be distinguished. Thus, in the anterior horn in the cervical and lumbar enlargements, a and a and a and a group are clearly recognized ; between these groups lies the intermediate zone or central field that borders the posterior horn.
Dorsal to the dorso-lateral cells, is situated the of the lateral horn, while Clarke' s column lies somewhat medial to the transition of the intermediate zone into the posterior horn.
Everywhere within the posterior horn, mostly small cells are scattered without the definite disposition of the anterior cornu ; nevertheless, also here different groups are defined, as the basal, the central and the marginal cells and those of the substantia Rolandi.
In contrast to this subdivision of the cord-cells according to position and arrange ment within the gray substance, a classification according to the behavior of their nerve processes is more appropriate for a presentation of the fibre-tracts. Therefore, we divide the cells of the spinal cord into : i. Cells, whose axones pass from the spinal cord. They lie within the anterior horn and are called the motor anterior Their nerve-processes form the anterior roots emerging from the spinal cord.
2. Cells, whose axones pass into the white substance. Within the latter, the axone divides into an ascending and a descending branch. The descending branch, after a short course, again enters the gray substance, where it ends; the ascending branch courses upward within the white substance. These cells are termed of which two varieties are distinguished : a. Cells, whose axones, that is, ascending branches, pass upward and as special paths connect the spinal cord with the brain; such are b. Cells, whose ascending branches or axones after a longer or shorter course
again enter the gray substance of the cord and serve to unite different cord-segments; such are The column-cells may be further distinguished as homolateral and contralateral. The axones of the former pass into the white substance of the same side, those of the latter to the white substance of the opposite side by way of the anterior commissure The column-cells are designated respectively as anterior, lateral or posterior column-cells according to the column in which they course. While the column cells are found in all parts of the gray substance, the contralateral column-cells occur chiefly within the base of the posterior horn and the intermediate zone. (Commissure cells are shown in the first and second cross-sections in Fig. 148. ) 3. Cells of Golgi II type. These are found predominatingly within the posterior horns and the substantia gelatinosa Rolandi.
The motor anterior whose axones form the motor anterior roots, occupy a special position, since they are the only elements that send their axones from the cen tral organ to the periphery. The column-cells and the Golgi cells, with their entire expansions, belong to the central nervous system. The establish relations between the spinal cord and the higher lying centres ; the association-cells serve to transfer an impulse received within the cord to higher • and lower lying cell-complexes ; while the field of activity of the cells of Golgi's II type is limited to the immediate vicinity. The nerve processes of the association-cells are also spoken of as endogenous fibres.