1032 



HUMAN ANATOMY. 



the axone divides into two, r.irdy three, fibres, one of which crosses by way of the anterior 

 white commissure to the opposite ventral column, while the other passes to the ventral 

 column of the same side. 



As well seen in cross-sections, although the nerve-cells of the anterior horn are widely 

 scattered they are nut uniformly distributed through the gray matter, but are collected into 

 more or less definite groups that recur in consecutive sections. It is evident, therefore, that the 

 cell-groups are nut limited to a single plane, but are continuous as longitudinal tracts or 

 columns for longer or shorter stretches within the core of gray matter of the cord. 



The grouping of the nerve-cells of the anterior horn includes two general 

 collections, a mesial group, containing many commissural cells, and a lateral group 

 composed chiefly of ventral radicular cells. These collections, however, vary in 

 extent and definition in different parts of the cord and, where well marked, are often 



FIG. 888. 



\ 



\ 



Cells of substantia 

 gelatinosa Rolandi 



Posterior horn cells 



Accessory dorso- 

 lateral groups 



Dorso-lateral group 



._> 



:.- 



Ventro-lateral group 



Mesial group 

 Transverse section of lower cervical cord, showing grouping of nerve-cells ; Nis>l staining. X to. 



made up of more than a single aggregation of cells. This feature is particularly evi- 

 dent in the lateral collection, in which an anterior and a posterior subdivision are 

 recogni/ed as the rfntro-laffm/ and the dorso-lati*ral ^ronp that occupy the corre- 

 sponding angles of the anterior horn. The mesial collection, situated within the 

 ventral angle, is likewise, but much less clearly, divisible into a rfHtro-wcsial and a 

 dorso-nir-sitil i; roup. of which the latter is variable and at many levels wanting. In a 

 r;il way the pronounced presence of these cell-groups influences the outline of 

 the anterior horn, so that corresponding projections of the gray matter mark their 

 position. This relation is conspicuously exemplified in the cervical and lumbo-sacral 

 enlargements, in which the presence of la rye lateral cell-groups is directly associated 

 with a marked increase in the transverse diameter of the anterior horn. Conversely, 

 when these cell-columns become smaller or disappear, the corresponding elevations 

 on the surface <>f the anterior horn diminish or are absent. Owing to such variations 

 the contours of tli <>re are subject to constant and sometimes abrupt change. 



