XI FUNCTIONS OF NERVE ROOTS 473 



contract, but pain is referred to the whole area of skin to 

 which the fibres of the nerve are distributed. 



It is clear enough, from these experiments, that all the 

 power of causinfj; muscular contraction which a spinal 

 nerve possesses is lodged in the fibres which compose 

 its anterior roots ; and all the power of giving rise to 

 sensation, in those of its posterior roots. Hence the 

 anterior roots are commonly called motor, and the pos- 

 terior sensory. 



The same truth may be illustrated in other ways. 

 Thus, if, in a living animal, the anterior roots of a spinal 

 nerve be cut, the animal loses all control over the nmscles 

 to which that nerve is distributed, though the sensibility 



3.jr 



Fig. 152.— Diagram to illi'stratk Experiments ik proof of the Func- 

 tions OF THE Spinal Nkkve-Roots and of the Gangliok on the 

 Posterior Root. 

 AF, anterior fissure of spinal cord ; PF, posterior fissure ; AR, anterior 



root of spinal nerve ; PR, posterior root ; T, trunk of spinal nerve ; 



Gn, ganglion of posterior rout. 



of the region of the skin supplied by the nerve is perfect. 

 If the posterior roots be cut, sensation is lost, and volun- 

 tary movement remains. But if both roots be cut, neither 

 voluntary movement nor sensibility is any longer possessed 

 by the part supplied by the nerve. The muscles are said 

 to be paralysed ; and the skin may be cut, or burnt, with- 

 out any sensation being excited. 



If, when both roots are cut, that end of the motor root 

 which remains connected with the trunk of the nerve be 

 irritated, the muscles contract ; while, if tlie otlier end be 

 so treated, no ajjparent effect results. On the other hand, 

 if the end of the sensory root connected with the trunk of 

 the nerve be irritated, no apparent effect is produced, 



