STRUCTURE OF THE NERVOUS CENTRES. 



455 



II. STRUCTURE OF THE NERVOUS CENTRES. 



The nervous centres are 



1. The spinal cord. 



2. The encephalon: consisting, ls, of the medulla oblongata and 

 the pons Varolii ; 2dly, the cerebellum ; Sdly, the cerebral ganglia ; 

 and, Stilly, the cerebral hemispheres. 



The centres consist of the white and the gray nerve-substance 

 combined ; the former being made up of nerve-fibres alone (and 

 vessels), the latter of nerve-cells and nerve-fibres combined, toge- 

 ther with (in the cerebellum and cerebral hemisphere) a granular 

 substance and free nuclei. The gray substance is also abundantly 

 supplied with vessels. The whole cerebro-spinal axis (i. e. the cen- 

 tres just mentioned) is also enveloped by three distinct membranes ; 

 which will be considered after the structure of the former has been 

 described (p. 468). 



Transverse section of human spinal cord through the middle of the lumbar enlargement, showing 

 on the right side the course of the nerve-roots, and on the left the position of the principal tracts of 

 vesicular matter. A, A. Anterior columns. P, P. Posterior columns. L, L. Portion of lateral 

 columns, a. Anterior median fissure, p. Posterior median fissure, b, b, b, b. Anterior roots of 

 spinal nerves, c, c. Posterior roots, d, d. Tracts of vesicular matter in anterior column, e. Tracts 

 of vesicular matter in posterior column. /. Spinal canal, not normal, g. Substantia gelatinosa. 



