COMPONENT PARTS OF GRAY MATTER OF SPINAL CORD. 429 
stantia gelatinosa stains more deeply with carmine, and has a more transparent or 
translucent appearance than the substantia spongiosa. 
In transverse sections of the cord the substantia Rolandi, in the cervical and 
dorsal regions, presents the appearance of a V-shaped mass, embracing the extremity 
of the caput of the posterior horn of gray matter; in the lumbar region this cap 
assumes a semi-lunar outline. 
In the substantia gelatinosa the neuroglia is present in large quantity, and in 
the Rolandic portion of it small nerve-cells are developed in considerable numbers. 
In the substantia centralis the nerve-cells are few in number, and the ring which it 
forms is traversed by the fine processes which proceed from the deep ends of the 
ependymal cells which line the central canal. 
Nerve-Cells.—The nerve-cells are scattered plentifully throughout the gray 
matter, but perhaps not in such great numbers as might be expected when we note 
the enormous number of nerve-fibres with which they stand in relation. They are 
all, without exception, multipolar, and send off from their various aspects several 
branching protoplasmic processes or dendrites, and one axon, which becomes the 
axis cylinder of a nerve-fibre. In size they vary considerably, and it is generally 
admitted that the bulk of a nerve-cell has a more or less definite relation to the 
length of the axis cylinder which proceeds from it. 
When the nerve-cells are studied in a series of transverse sections of the cord, 
it will be noticed that a large proportion of them are grouped in clusters in certain 
districts of the gray matter ; and as these groups are seen in very much the same 
position in successive sections, it is clear that these cells are arranged in longitudinal 
columns of greater or less length. Thus we recognise (1) a ventral group or 
column of cells in the anterior horn of gray matter ; (2) a lateral group or column 
in the lateral horn of gray matter, where this exists; and (3) a posterior vesicular 
column of cells (Clarke's column), forming a most conspicuous group in the mesial 
part of the cervix of the posterior horn in ‘the dorsal region of the cord. 
Other cells besides those forming these columns are “scattered somewhat irregu- 
larly throughout the gray matter of the posterior horn and the part of the gray 
crescent which lies between the two horns; and although these also in some measure 
may be classified into groups, the ar rangement thus effected is not of so definite a 
character as to justify us in dwelling upon it in the present instance. 
Ventral Cell-Column and the Origin of the Fibres of the Anterior Nerve- 
Roots.—The ventral cell-group occupies the anterior horn of gray matter, and in 
it are found the largest and most conspicuous cells in the spinal cord. It extends 
from one end of the cord to the other. These ventral nerve-cells have numerous 
wide-spreading dendritic processes, and it is to be noticed that certain of these 
dendrites do not confine their ramifications to the gray matter. Thus, some of the 
cells along the mesial border of the anterior horn of gray matter send dendrites 
across the mesial plane in the anterior commissure to end in the anterior gray 
horn of the opposite side; whilst others, lying along the lateral or outer margin of 
the anterior horn of gray matter, send dendrites in amongst the nerve-fibres of the 
adjoining white matter. 
The axons or axis cylinder processes of a large proportion of the ventral cells 
converge together ; and, becoming medullated, form bundles which pass out from 
the gray matter, and through the white matter which separates the thick end of 
the anterior horn from the surface of the cord, to finally emerge as the fascicles of 
the anterior nerve-roots. These cells, then, are the sources from which the nerve- 
fibres of the anterior nerve-roots proceed, and in consequence they are frequently 
spoken of as the “ motor cells” of the cord. 
In the chick a few of the more deeply seated of the ventral cells, placed in the 
cervical part of the anterior horn of gray matter, send their axons backwards through the 
posterior horn to the postero-lateral groove. Here they emerge from the cord and con- 
stitute a small group of efferent nerve-fibres in the afferent posterior nerve-root. It is 
more than doubtful if any such fibres exist in the posterior nerve-root of the mammal 
(Sherrington). 
The ventral cells are not scattered uniformly throughout the anterior horn of gray 
matter. They are aggregated more closely together in certain parts of the anterior horn, 
