THE NERVOUS SYSTEM 221 



23. If a nerve that has been exposed be divided, and the 

 inner end, or that still in connection with the nerve-centres be 

 irritated, sensation is produced, but no movement takes place. 

 But if the outer end, 



or that still connected 

 with the limb, be irri- 

 tated, then no pain is 



felt, but muscular con- ^JB)^^ />^ ,/ 

 tractions are produced. 



Thus we prove that FlG . 60 . _ ILLUSTRATING TIIE Ft:NCTION9 OF THE 

 there are two distinct SPINAL NERVES 



Sets Of fibres ill the Divided at a. Irritated at 1 : pain. Irritated at 2 : 



muscular contraction 



nerves one of which, 



the sensory fibres, conduct toward the brain, and another, the 

 motor fibres, conduct to the muscles (see Fig. 60). The 

 former may be said to begin in the skin and other organs 

 and end in the brain, while the latter begin in the nervous 

 centres and end in the muscles. They are like a double line 

 of telegraph wires, one for inquiries, the other for responses. 



24. We have already spoken of the two roots of the spinal 

 nerves, called, from their points of origin in the spinal cord, 

 the anterior and posterior roots. These have been separately 

 cut and irritated in the -living animal, and it has been found 

 that the posterior root contains only, sensory fibres, and the 

 anterior root has only motor fibres. So that the nerves of a 

 limb may be injured in such a way that it will retain power of 

 motion and yet lose sensation ; or the reverse condition, feeling 

 without motion, may exist. Between these two sorts of fibres 

 no difference of structure can be found ; and where they have 

 joined to form a nerve, it is impossible to distinguish one sort 

 from the other. 



25. Occasionally a nerve is so compressed as to be tempo- 

 rarily unable to perform its functions : a transient paralysis 



23. If an exposed nerve be divided ? What is proved ? The course of the sensory set 

 of fibres ? Of the motor set ? To what are they likened ? 



24. The two roots of the spinal nerves ? What has been found ? Difference of the two 

 sorts of fibres ? Result of their union ? 



25. Transient paralysis? When such is the case with the leg? What other fact is 

 observed ? 



