THE CENTRAL NERVOUS SYSTEM 



673 



tions on opposite sides of the middle line (commissural 

 fibres). Our first task is, therefore, to trace the peripheral 

 nerves to their cells of origin or centres of reception* in the 

 nervous stem. And although there is reason to believe that, 

 the whole of the peripheral nerves, cerebral and spinal 

 (with the exception of the olfactory and optic, which are 



En lei rg 



L : 



Sfillin f/s Ct-' r vi cal 

 micleu s 



i L ate ra/ cell- col urn n 



-- (column cf the tnter- 

 j medic-lateral tract) 



Stiilin <y 's a f t> rs a I 

 n ticleus vrClar/rt's 

 Col umn. 



Lumbar 



Enlarement 



1-f Scattered cells of 

 inte r me die -lateral 

 tract. 



t tiling's Sacral 

 n uclt us 



FIG. 238. DIAGRAM OF GREY TRACTS OF CORD. 



rather portions of the brain than true peripheral nerves), 

 form a morphological series, it will be well to begin with 

 the spinal nerves, since their motor and sensory fibres 

 are gathered into different and definite roots, whose course 

 within the cord is, in general, more easily traced than the 

 course of the cerebral root-bundles within the brain. 



* The centre or nucleus of reception of a nerve contains the nerve-cells 

 around which its axons terminate ; the nucleus of origin of a nerve con- 

 tains the cells from which its axons arise. 



43 



