EESTEVEN ON THE ANATOMY OF THE SPINAL COED. 391 



to those which are observed in ascending to the cervical enlargement. 

 The posterior cornua become separated in a direction obliquely 

 backwards. At the upper part of the lumbar enlargement the 

 posterior vesicular columns are decidedly larger than in any other 

 part of the cord. Its larger cells are more numerous, and do not 

 form a circumscribed group, but lie scattered through the whole inner 

 half of the cervix which their processes traverse in different direc- 

 tions and planes. Through the remaining half of the lumbar enlarge- 

 ment, the posterior vesicidar columns are gradually less encircled by 

 their processes, which are also fewer in number, but are still traversed 

 by the divergent fibre of the transverse commissure, and by a plexus of 

 the posterior roots which sweep round on their outer sides. 



In descending the lower portion of the lumbar enlargement, the 

 border of the grey substance between the posterior cornua is 

 gradually drawn backwards, so that in the same proportion the space 

 behind the canal, containing the transverse commissure becomes 

 deeper. 



Near the level of the second pair of sacral nerves is a pecuMar 

 group of cells, which was pointed out by Clarke in 1851, and was 

 regarded by him as the commencement of the posterior vesicidar 

 column, but that anatomist now couciu"s with Stilling in regarding 

 it as a distinct group. It is more or less oval, but is not entirely 

 isolated from surrounding cells. The principal part of the group is 

 intimately connected with the anterior roots of the nerves. 



Dean, who had independently examined the cells of the horns in 

 the lumbar region before he had seen Clarke's paper (1859), observes, 

 that his "observations are entirely in agreement with his state- 

 ments iu all important particulars." 



Tlie central canal, as first exactly described by Clarke ia 1851, 

 is lined with columnar epithelium, and ia the ox, with fusiform cells 

 also. Between these two kinds of epitheKum there are different 

 grades of transition. They are aU beautifully packed in close 

 apposition, so that the convexity of each is applied to the concavity 

 of those which surround it. In the human cord the canal is often 

 filled with what would appear to be the debris of epithelium, for 

 nothing is to be seen but a confused heap of nuclei ; but sometimes 

 in the midst of this heap there remains a small opening or canal, 

 and sometimes two such secondary canals, each being lined by a 

 regular layer of columnar cells. The cilia of the epithelium are 

 coarser and less numerous than those in the larnyx and trachea. 

 The light coloured space surrounding the canal is interspersed with 

 nuclei, or minute cells. Some of them are oval or round, finely 

 granular, and exactly resemble those in the connective tissue of the 

 white columns. 



KoUiker formerly denied the existence of a central canal, but is 

 quoted by Lenhossek as having admitted its existence. 



" Hannover," observes Clarke, " regards the cells which line the 

 cerebral ventricles as true nerve-cells, and Bidder takes the same view 



