Chap. VII.] 



THE NERVOUS SYSTEM. 



83 



Cervical 



Dorsal 



Lumbar 



Sacral 



Coccygeal 



6 pairs 

 12 " 

 5 " 

 5 " 



1 pair 



^^hr^ 



The spinal nerves pass out of the spinal canal through the 

 intervertebral foramina, the openings between the vertebroe 

 spoken of in the lesson on the bones of the spine. 



Each spinal nerve has two roots, a ventral root and a dorsal 

 root. The fibres connected with these two roots are collected 

 into one bundle, and 

 form one nerve just — ■ ^ '^ 



before leaving the canal 

 through the interverte- 

 bral openings. Before 

 joining to form a com- 

 mon trunk, the fibres 

 connected with the dorsal 

 root present an enlarge- 

 ment, this enlargement 

 being due to a ganglion, 

 or small nerve-centre. 

 The fibres of the ventral 

 root arise from the gray 

 matter in the ventral 

 horn, and are direct pro- 

 longations from the cell- 

 bodies there. The fibres 

 of the dorsal root, on the other hand, arise from the cell-bodies 

 in the ganglion, and ^row into the nerve-centres forming the gray 

 matter in the dorsal horn. All the fibres growing from the ven- 

 tral root are efferent fibres, and convey nervous impulses from 

 the spinal cord to the periphery. The fibres growing into the 

 dorsal root are afferent fibres, and convey nervous impulses from 

 the periphery 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. This is true also of the 



Fig. 71. — Diagram showing Anatomy of 

 THE Spinal Nerve Roots and Adjacent 

 Parts. G., gray matter of the spinal cord; W., 

 white matter of the same; D.H., dorsal horn of 

 gray matter; F.//., ventral horn of gray matter ; 

 D.R., dorsal root of spinal nerve; Sp. ^., spinal 

 ganglion ; V.R., ventral root of spinal nerve ; Sp. N., 

 spinal nerve; i^c, communicating branch {ramus 

 comniunicaus) ; S.G., sympathetic ganglion. 



