106 NERVOUS SYSTEM. 



lateral and posterior columns of the white substance, and the names 

 anterior and posterior horns (cornua) are given. to these regions 

 respectively. 



What are the nerve-roots of the cord ? 



Issuing from the cord along its course are thirty-one pairs of 

 nerves. Each of these spinal nerves is made up of an anterior and 



a posterior root (Fig. 28), of 

 FIG. 28. which the latter is the larger. 



The anterior root arises between 

 the anterior and lateral white 

 columns, the posterior between 

 the posterior and lateral col- 

 umns. On each posterior nerve- 

 Transverse Section of the Spinal Cord : a, 6, root is found a ganglion imme- 



spinal nerves of right and left sides; Jiately beyond its point of 

 rf, origin of anterior root; e, origin of pos- * J _.. _ " . 



terior root ; c, ganglion of posterior root, emergence. Ihe tunction OT 



this seems to be trophic. 



What is the minute structure of the white substance ? 



It is found to be made up of medullated nerve-fibres, which 

 collect to form the anterior and posterior nerve-roots, and commu- 

 nicate with other regions of the cord. 



What is the minute structure of the gray matter ? 



The gray matter contains multipolar cells of varying size and 

 shape, with axis-cylinders and " branching " processes lying in 

 the neuroglia (connective tissue). The multipolar cells are some 

 of them quite large. In the anterior horn of the gray substance 

 the axis-cylinder-processes of the nerve-cells connect directly with 

 fibres forming the anterior nerve-roots (Fig. 29) ; but in the poste- 

 rior cornu the communication is through the branching processes 

 joining the divided axis-cylinders of the posterior nerve-roots, form- 

 ing thus a minute plexus known as Gerlach's nerve-network. 



Whence are the fibres derived which make up the nerve-roots ? 

 (a) Anterior nerve-roots are derived from (1) the anterior col- 

 umns of the cord, but some fibres come through the commissure 

 from (2) the opposite side, and some come from (3) the lateral 

 tract. Still other fibres arise from (4) the multipolar cells in the 

 anterior cornu of the gray matter. The fibres of the anterior 

 nerve-roots are efferent or motor fibres. 



