350 THE NERVE SYSTEM 



All spinal nerves leave the spinal canal between the 

 vertebra (inter vertebral foramen). If one examines 

 the spinal cord after removal, the spinal nerves will 

 be seen to consist of two roots by which they arise 

 from the sides near the anterior and posterior aspects 

 of the cord. The two roots are named anterior or 

 ventral and posterior or dorsal. The anterior and 

 posterior roots join to form a single nerve trunk just 

 before they leave the spinal canal. The dorsal root 

 presents an enlargement near the point at which it 

 joins the anterior root a small grayish body called 

 a ganglion. The roots do not leave the cord as a 

 single rounded nerve, but are formed by the joining 

 of four to six large nerve fibers. 



FIG. 125 



DORSAL RO 



INAL ROOT GANG 



IL NERVE 



VENTRAL FISSURE 



Showing origin of two pairs of spinal nerves (schematic). (Gray.) 



The posterior root is sensor or afferent, and contains 

 bundles of axones which convey impulses from the 

 end-organs in the skin, mucous membranes, etc., 

 after they have been received and conveyed by axones 



