SECT. III.] NERVOUS SYSTEM. 21 9 



in a transverse section, it forms a cross. The middle portion, or 

 grey commissure, contains, in the foetus, and, in most cases, also in 

 the adult, a canal (canalis spinalis), with cylindrical epithelium, 

 and, around this, a grey substance, the central grey nucleus, as it 

 was termed by me, but which, with Virchoiv, I reckon the epen- 

 dyma of the canalis spinalis, and will call central ependymatic 

 filament. Before and behind this filament there are nerve-fibres 

 running in the transverse direction, the grey or posterior commis- 

 sure. Of the grey lamina?, also called horns (corny a), the anterior 

 are shorter and thicker, uniformly grey, and composed of larger 

 and smaller, nerve- cells and delicate moderately-fine nerve-fibres ; 

 the posterior are longer and narrower, and, at their origin, con- 

 structed like tlu> anterior, but mostly with smaller cells ; at the 

 free border, on the other hand, they are covered by a clearer layer, 

 composed principally of small cells, substantia gelatinosa (Rolando). 

 With regard to the roots of the spinal nerves, the anterior pass 

 between the anterior and lateral columns direct to the anterior 

 cornua ; and the posterior, passing through the suhstantia gela- 

 tinosa into the posterior laminse, are lost between the lateral and 

 posterior columns. 



With regard to the internal structure of the spinal marrow, 

 there are to be distinguished in the white substance, 1, horizontal, 

 and, 2, longitudinal fibres. The latter are found in all places with 

 the exception of the anterior commissure, are, for the most part, 

 unmixed with horizontal fibres, and run everywhere parallel to 

 each other, without even forming plexuses or constituting fine 

 bundles. These fibres, of 0-0012'" to 0-0048"' in thickness, decrease 

 in number from above downwards, while, as will be afterwards 

 shown, they successively pass into the grey substance, and present 

 the general characters of central nerve-tubes. The transverse fibres 

 are found, 1, in the parts of the lateral and posterior columns 

 adjoining the cornua of the grey substance, the description of 

 which will be given below when treating of that substance ; 

 2, in the white commissure ; and, 3, at the place of entrance of 

 the roots of the nerves. The ichite or anterior commissure is 

 only in part a commissure, in the usual acceptation of the term, 

 inasmuch as it is formed, in part, by the deepest nerve-fibres of 

 the anterior columns, which, whilst they bend inwards in an 

 oblicme direction, decussate in front of the grey commissure, — the 

 bundles coming from the right anterior column spreading out hori- 

 zontally in the left anterior horn of the grey substance, and those 

 from the left column radiating into the right anterior horn. Other 

 fibres of this commissure arc real commissural filaments, and 



