140 THE NERVOUS SYSTEM. 



bundles towards the periphery. The transverse fibres are arranged 

 chiefly in the two commissures : the dorsal commissure [coinmissura 

 superior) is the smaller; the fibres are parallel, and show no trace 

 of decussation ; externally they radiate in various directions to the 

 dorsal horns. The ventral commissure is composed of two layers^ the 

 upper grey {commissvra inferior grisea), the lower white [coriinmsura 

 inferior alba) ; both are interrupted by the septum medium. A well- 

 marked decussation of the fibres is seen in the middle line ; the exact 

 mode of termination of these fibres has not been made out, but many 

 appear to communicate with the large cells of the ventral horns. 



The white matter consists chiefly of longitudinal, medullated 

 fibres, in which various columns may be distinguished. The dorsal 

 columns are separated from the lateral by a process of nem-oglia, 

 continued from the general investment of neuroglia lying under the 

 pia 7nater. The line of separation between the lateral and ventral 

 columns is not well marked ; it is about the line which would be 

 formed by prolonging the ventral horns to the surface. 



The fibres vary in size, and fibres of all sizes may be found 

 in any particular part of a transverse section ; still the fibres of the 

 ventral columns have an average greater diameter than those of 

 the lateral columns, and the fibres of the dorsal columns are finer 

 than those of the lateral. The largest fibres of the ventral column 

 are placed near the ventral fissure and on its lower border; they 

 attain their greatest development in the posterior enlargement; 

 these fibres frequently contain two or three axis -cylinders each. 

 Koppen suggests that they may represent theformatio reticularis of 

 higher animals. 



In the lateral columns the larger fibres are placed close to the 

 grey matter, these columns also receive fibres from the cells of the 

 lateral group. 



In the dorsal columns the radiating root- fibres never reach the 

 median plane, but leave an area of purely longitudinal fibres on each 

 side of the dorsal fissure ; these represent GolFs columns, and have 

 a club-shaped outline in transverse section. 



The white matter is pierced in all directions by fine connective- 

 tissue fibres and bundles of fibres which radiate from the grey matter; 

 some branch and join with others to form a network, others pass 

 almost uninterruptedly to the periphery. From the connective-tissue 

 cells of the/)?a mater, processes pass into the white matter and assist 

 in completing the connective-tissue matrix for the nervous elements. 



Dorsal roots of the spinal nerves. Each root consists of a 



