480 THE NERVOUS SYSTEM 



The spinal cord may be considered as consisting of ontogenetic 

 segments whose number corresponds to the number of the spinal 

 nerves. Hence each segment contains the anterior horn cells 

 whose neuraxes form the ventral root of the corresponding spinal 

 nerve. 



In an entirely similar manner the posterior horns of the grey 

 matter receive a large portion of the incoming fibres of the pos- 

 terior roots, which in large part form end brushes around the cells 

 of the dorsal horns and the intermediate zone. 



The dorsal roots enter through a distinct longitudinal groove, 

 the postero-lateral sulcus. At the exit of the ventral roots there 

 is, however, only a broad shallow indentation, these roots making 

 their exit in isolated bundles distributed through a vertical plane 

 of considerable width. The dorsal root fibres of each segment, on 

 the other hand, enter in a single compact mass. 



The grey matter consists of a dense tangle of nerve cells and 

 fibrils, together with neuroglia and blood vessels. The fibrils of a 

 given area are derived not only from nerve cells in their imme- 

 diate vicinity, but also include many processes which come from 

 very distant regions. The grey reticulum is thus supplied from 

 fibres of the ventral and dorsal nerve roots, together with innu- 

 merable collaterals, not only from the root fibres, but more espe- 

 cially from those fibres which collectively form the many large 

 tracts passing up and down the spinal cord and placing each seg- 

 ment in communication with many other levels of both the spinal 

 cord and brain. 



The center of the grey commissure contains the central canal 

 which lies in the axis of the spinal cord and is continuous above 

 with the ventricles of the brain. It represents the remains of the 

 fetal neural canal ; and in the young subject is still patent, filled 

 with cerebro-spinal fluid, and lined by columnar cells which are 

 frequently ciliated. In older subjects the cells of the lining epithe- 

 lium have usually lost their cilia, and the lumen of the canal is more 

 or less filled by cell proliferation which involves not only the lining 

 epithelium but also the surrounding glia cells and fibroblasts. 



The central canal is immediately surrounded by a peculiar 

 gelatinous tissue in which are many glia cells. This mass is called 

 the substantia gelatinosa centralis. A similar area of gelatinous 

 tissue occurs near the dorsal extremity of the posterior horns, and 

 is called the substantia gelatinosa posterior or gelatinous substance 

 of Rolando. 



