528 THE NEKVOUS SYSTEM. 



neuroglia, whilst the nerve -fibres traverse fine passages the walls of which ar 

 formed of the same substance. The neuroglia is thus an all-pervading basis sub 

 stance which isolates the nervous elements one from the other more or less com 

 pletely, and at the same time binds them together into a consistent solid mass 

 In two situations the gray matter presents peculiar features, viz., the apex of th< 

 posterior column and the tissue surrounding the central canal. In both situation 

 the gray matter stains more deeply with carmine and presents a more translucen 

 appearance; in other respects the substantia grisea centralis and the substanti, 

 gelatinosa are very different. 



The substantia grisea centralis forms a thick ring around the central cana 

 It is traversed by the fine processes which proceed from the deep ends of th 

 ependymal cells which line the canal. It is composed almost entirely of neuroglie 



In transverse sections of the spinal medulla the substantia gelatinosa, in th 

 cervical and thoracic regions, presents the appearance of a V-shaped mast 

 embracing the extremity of the posterior column of gray matter ; in the lumba 

 region this cap assumes a semilunar outline. 



In the substantia gelatinosa the neuroglia is present in small quantity, an 

 small nerve-cells are developed within it in considBrable numbers. 



Nerve-Cells. The nerve-cells are scattered plentifully throughout the gra 

 matter, but perhaps not in such great numbers as might be expected when we not 

 the enormous number of nerve-fibres with which they stand in relation. They ar 

 all, without exception, multipolar, and send off from their various aspects severe 

 branching protoplasmic processes or dendrites, and one axon, which becomes th. 

 axis-cylinder of a nerve- fibre. In size they vary considerably, and as a rule (t 

 which, however, there are many exceptions) the bulk of a nerve-cell has a mor 

 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 spine 

 medulla, it will be noticed that a large proportion of them are grouped in clusters i 

 certain districts of the gray matter ; and as these groups are seen in very much th^ 

 same position in successive sections, it is clear that these cells are arranged in long: 

 tudinal columns of greater or less length. Thus we recognise (1) a ventral grou 

 'or column of cells in the anterior column of gray matter ; (2) an intermedio-laten 

 group or column in the lateral column of gray matter, where this exists ; and (3) 

 posterior vesicular column of cells (nucleus dorsalis), forming a most conspicuou 

 group in the medial part of the neck of the posterior column in the thoraci 

 region of the spiual medulla. 



Other cells, besides those forming these columns, are scattered somewhat irregi 

 larly throughout the gray matter of the posterior column and the part of the gra 

 crescent which lies between the two columns; and although these also in som 

 measure may be classified into groups, the arrangement thus effected is not of s 

 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 Nerv* 

 roots. The ventral cell-group occupies the anterior column of gray matter, and i 

 it are found the largest and most conspicuous cells in the spinal medulla. ] 

 extends from one end of the spinal medulla to the other. These ventral nerve-ceL 

 have numerous wide-spreading dendritic processes, and it is to be noticed tha 

 certain of these dendrites do not confine their ramifications to the gray matte 

 Thus, some of the cells along the medial border of the anterior column of gra 

 matter send dendrites across the median plane in the anterior commissure to en 

 in the anterior gray column of the opposite side; whilst others, lying along th 

 lateral margin of the anterior column of gray matter, send dendrites in among? 

 the nerve- fibres of the adjoining white matter. 



The axons or axis-cylinder processes of a large proportion of the ventral cells coi 

 verge together ; and, becoming medullated, they form bundles which pass out froi 

 the gray matter, and through the white matter which separates the thick end < 

 the anterior column from the surface of the spinal medulla, to emerge finally i 

 the fila of the anterior nerve-roots. These cells, then, are the sources from whic 

 the nerve- fibres of the anterior nerve-roots proceed, and in consequence they ai 

 frequently spoken of as the " motor cells " of the spinal medulla. Whilst this 



