THE SPINAL CORD. 139 



by meclullated fibres. This septttm medium (Reissner) may be traced 

 backwards from the anterior extremity of the cord to the posterior 

 enlargement. It consists chiefly of transverse fibres, tog-ether mth 

 vertical fibres derived from the epithelial cells on the floor of the 

 canal, and like the substantia reticulosa, is distinguished by its 

 greater transparency from the rest of the grey matter. 



The grey matter consists of connective-tissue and nervous ele- 

 ments, which are very intimately united; each of these elements 

 including its special cells and fibres. The nerve-cells vary much in 

 size and appearance. A prominent group of large cells in the ven- 

 tral cornua, the lateral group (Stieda), is very conspicuous. The 

 individual cells are seldom rounded, but are usually spindle-shaped 

 or angular, and each possesses one to five processes, which may 

 often be traced through considerable distances. These cells have an 

 average length of 0*040 mm., and are about 0'0i6 mm. broad; 

 they are somewhat larger in the anterior enlargement than in the 

 posterior, and are also more numerous in a section from the anterior 

 enlargement than in a section from the posterior. In the terminal 

 filament they gradually disappear. Some of their processes are 

 continued into the lateral columns (Koppen). 



Smaller, spindle-shaped or triangular cells are scattered irregu- 

 larly throughout the grey matter, without forming distinct groups, 

 though for descriptive purposes those of the centre have been named 

 the central group. In a stained, transverse section of the cord are 

 seen numerous nuclei; these undoubtedly belong both to small 

 nerve-cells and to connective-tissue cells, but except under the most 

 favourable conditions the two are indistinguishable. The larger 

 nerve-cells present the usual characters of nerve-cells : more or less 

 granular contents, nucleus, well-marked nucleolus, more or less 

 marked fibrillation ; their processes are usually more homogeneous. 

 The cells are frequently j)igmented. 



The processes of the cells belonging to the lateral group radiate 

 in all possible directions, but certain well-marked processes directed 

 towards the middle line can always be made out. From the lowest 

 part of this group a few processes may be traced into the ventral 

 roots. The small cells seem to be chiefly arranged vertically, al- 

 though their processes radiate in all directions (Stiedaj. 



The Fibres of the grey substance are nearly all non-medullated. 

 They may be traced in all directions, but the best-marked groups 

 are either vertical or transverse. The vertical fibres appear to arise 

 from the central grey matter, and to ascend in larger or smaller 



