THE NERVOUS SYSTEM. 161, 



cular contractions are produced. Thus we prove that there 

 are two distinct sets of fibres in the nerves ; one of which, 

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

 the motor fibres, conduct to the muscles. 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. Be- 

 tween 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 tem- 

 porarily unable to perform its functions : a transient par- 

 alysis then takes place. This is the case when the leg or 

 arm "gets asleep," as it is expressed. When such is the 

 condition with the leg, and the person suddenly attempts 

 to walk, he is liable to fall, inasmuch as the , motor fibres 

 cannot convey orders to the muscles of the limb. Another 

 fact is observed : there is no sensation in this nerve at the 

 point of its compression ; but the whole limb is numb, and 

 tingling sensations are felt in the foot, the point from 

 which the sensory fibres arise. 



26. This illustrates the manner in which the brain in- 



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 i e observed ? 



26. What does this illustrate ? Sensation ? The feeling after a limb has been 

 amputated ? Striking of the u funny bone ?" 



