138 THE NERVOUS SYSTEM. 



I distinct lumen ; in the two enlargements this lumen forms a vertical 

 ellipse x , while in the pars media it is circular. The canal is lined by 

 a single layer of ciliated columnar epithelium, the cells of which 

 are usually conical, with their bases directed towards the lumen ; but 

 other forms may be noticed. These cells average about 0-040 mm. 

 in length and about 0-002 mm. in width at their broadest part. 

 The peripheral extremities of the cells have processes, which, in the 

 case of the cells above and below the cord, may often be traced 

 as far as the pia mater ; the processes of the lateral cells are less 

 distinct and seem to be shorter. Anteriorly the central canal 

 opens into the floor of the fourth ventricle. The termination of 

 the central canal, posteriorly, has been described by Masius 

 and Vanlair. According to these observers, the canal towards the 

 hinder end of ihejilum terminate increases very gradually in width, 

 until its epithelial cells are immediately underneath the pia mater, 

 its walls then converge somewhat more sharply to close the cavity. 

 From the point where the canal commences to increase in width, 

 a change takes place in its epithelial wall, which, instead of having 

 a single layer of cells, now possesses two to three layers ; towards 

 the posterior end it is again thinned out to a single layer. 



The grey matter varies in different parts of the cord and in dif- 

 ferent parts of the same section. In a section through the anterior 

 enlargement a portion of the grey matter, placed immediately above 

 the central canal, is easily distinguished by its vertically-placed, 

 elliptical outline, and by its transparency : it is known as the sub- 

 stantia reticularis. It consists of a mesh-work of branched cells, 

 through which course distinctly contoured fine commisural fibres 

 which are derived chiefly from the dorsal cornua, the vertical 

 from the epithelial cells of the roof of the central canal. In a 

 series of sections the substantia reticulosa may be seen to originate 

 indistinctly in the most anterior part of the cord ; in the anterior 

 enlargement it already possesses the elliptical form mentioned, and 

 in the pars media it commences to send off lateral processes to either 

 side. In the posterior enlargement it reaches its greatest absolute 

 development, as it here surrounds the central canal ; while further 

 backwards it encroaches until, together with the remaining grey 

 matter, it constitutes almost the whole of thejilum terminate. 



Immediately underneath the central canal is a narrow band, 

 which immediately connects the grey matter of the two sides ; 

 above it is bounded by the epithelium of the central canal, below 



1 v. Kolliker states that the lumen is everywhere circular. 



