THE SPINAL CORD 



189 



Thus we learn that the anterior roots consist of efferent or motor 

 fibres only. 



2. If the posterior root of the nerve supplying a certain part 

 is similarly injured, the animal will still have control over the 

 voluntary muscles of that part just as if nothing had happened, 

 but the part will have lost all power of sensation. Again, if the 

 root is divided, any irritation applied to the portion in contact 

 with the cord will 

 cause more or less 

 pain; but no effect is 

 produced by the irrita- 

 tion of the other part. 

 Therefore we conclude 

 that the posterior roots 

 consist of sensory or 

 afferent fibres only. 



Similar observa- 

 tions teach us that the 

 spinal nerves, formed 

 by the union of the 

 anteiior and posterior 

 roots, are mixed nerves, 

 that is, nerves com- 

 posed of both sensory 

 and motor fibres. For, 

 if the spinal nerve sup- 

 plying a certain part 

 of the body is injured 

 to such an extent that 

 its fibres are severed, 

 both sensation and 

 voluntary movement 

 are lost to that part. 

 Further, any irrita- 

 tion received by that 

 portion of the nerve 

 still in communication with the cord will produce sensation ; 

 while irritation to the other portion will cause muscular 

 contraction. 



It is an interesting fact that, when the portion of a divided spinal nerve 

 which remains in communication with the brain is irritated, or, when the 

 corresponding portion of a divided sensory root is irritated, the sensation is 

 not felt at the point where the irritation is applied, but is always referred to 

 the part in which the fibres of the divided nerve or root terminate. Thus, if 



Fig. 174. Illustrating the Functions of the 

 Boots of the Spinal Nerves. 



a, anterior root ; p, posterior root. 



Divided a t a. Irritated at i : no result. Irritated 

 at 2 : contraction of muscles supplied with fibres from 

 the root. 



Divided at /.Irritated at 3 : no result. Irritated 

 at 4 : pain produced. 



/ 



Fig. 175. Illustrating the Functions of the 

 Spinal Nerves. 



Divided at a. Irritated at i : pain. Irritated at 2 : 

 muscular contraction. 



