THE NERVOUS SYSTEM. 107 



tion with its prolongations, the comma, or horns, is formed, in 

 almost equal proportions, of nerve-fibres and cells. 



The white substance of the spinal cord may, for the purpose 

 of description, be most conveniently, and in accordance with 

 usage, divided into two halves, and each of these into three 

 columns. The anterior columns [funiculi anteriores), are, 

 towards the interior, almost completely separated from each 

 other by the anterior fissure [fissura anterior), which extends 

 the whole length of the cord, and into which a vascular pro- 

 cess of the pia mater penetrates. At the bottom of the fissure, 

 however, the columns are united by the anterior, or white com- 

 missure [com. alba) ; externally, they extend as far as the points 

 of exit of the anterior roots of the nerves, or to the sulcus 

 lateralis anterior, but are here inseparably connected with the 

 lateral columns [funiculi laterales), which again, at the points 

 of exit of the posterior roots, where the sulcus lateralis 

 posterior is situate, are continuous, without any line of demar- 

 cation, with the posterior columns. The latter [funiculi pos- 

 tei'iores) appear indeed as if they were in contact in the 

 posterior mesial line, because the posterior longitudinal fissure 

 described by many anatomists, does not exist in man, except 

 in the lumbar enlargement of the cord, and in the superior 

 cervical region ; but they are nevertheless separated, to such 

 a degree, throughout the whole length of the cord by very 

 numerous vessels, which in the posterior mesial line penetrate 

 as far as the grey nuclear portion, that the columns in 

 most places are not even in contact, and even where they are, 

 they are merely in juxtaposition, and never by any means 

 continuous into each other. Thus the white substance of the 

 cord represents two halves, united only by the anterior white 

 commissure, and each of which is divided more artificially into 

 three columns, which occupy the depressions left between the 

 projecting processes of the grey substance. 



The grey substance presents a central portion, more of a 

 riband-like form, and four lamina? projecting laterally from it, 

 so that its transverse section forms a cross. The central por- 

 tion or the grey commissure, in the adult, does not, normally, 

 contain any canal, such as exists in the foetus, and consists of a 

 central, cylindrical, or flattened tract, constituted principally 

 of nerve-cells, of a yellowish colour, — the grey nucleus 



