Chap. XIX] 



THE NERVOUS SYSTEM 



381 



trunk, the fibres connected with the dorsal root present an en- 

 largement, this enlargement being due to a ganglion, or small 

 nerve-centre, situated in the intervertebral foramina. The fibres 

 of the ventral root arise from the gray matter in the ventral horn, 

 and are direct prolongations from the cell-bodies there. 



The fibres of the posterior root arise from the cells composing 

 the enlargement or ganglion of the posterior root and pass toward 

 the periphery ; each cell of the ganglion, besides sending toward 



Fig. 181. — Diagram of Nerve Roots emerging from Spinal Cord. P.R. 

 posterior root. Sp.G. ijosterior root ganglion. A.R. anterior root. (Schematic.) 

 (Collins.) 



the periphery the nerve-fibres just described, sends a branch along 

 the posterior root up into the gray matter of the posterior 

 horn, there to break up into branches articulating with other 

 cells or dendrites. All the fibres making up the ventral root 

 are efferent fibres, and convey nervous impulses from the spinal 

 cord to the periphery. The fibres making up the dorsal root 

 are aflFerent fibres, and convey nervous impulses from the periph- 

 ery to the spinal cord. 



It should be borne in mind that the dorsal roots contain only 

 sensory fibres, and that these fibres always have their origin 

 outside of the cord (i.e. in the spinal ganglia), while the ventral 

 roots contain only motor fibres, and these have their origin 

 within the central nervous system. 



The relations of the roots, fibres, and so forth, can be best 



