222 THE NERVOUS SYSTEM 



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 inter- 

 prets all injuries of the trunk of a nerve. Sensation or pain 

 is not felt at the point of injury, but is referred to the outer 

 extremities of the nerve, where impressions are habitually 

 received. This is the reason why, after a limb has been 

 amputated by the surgeon, the patient appears to suffer pain 

 in the member that has been severed from the body; while 

 some form of irritation at the end of the nerve in the wound, 

 or stump, is the real source of his distress. Again, when thb 

 " funny-bone " that is, the ulnar nerve at the elbow is 

 accidentally struck, the tingling sensations thus produced are 

 referred to the outer side of the hand and the little finger, the 

 parts to which that nerve is distributed. 



27. All the spinal nerves, and two from the brain, are con- 

 cerned in both sensation and motion. Of the remainder of 

 the cranial nerves, some are exclusively motor, others exclu- 

 sively sensory; and still others convey, not ordinary sensa- 

 tions, but special impressions, such as sight, hearing, and 

 smell, which we have yet to consider. However much the 

 functions of the nerves seem to vary, there is but little dif- 

 ference discoverable in the nerves themselves, when examined 

 under the microscope. Whatever difference exists must be 

 accounted for in consequence of the nerves communicating 

 with different portions of the gray matter of the brain. The 

 rate of motion of a message, to or from the brain along a 



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

 tated ? Striking of the " funny-bone ? " 



27. The spinal nerves, and two from the brain ? Of the remainder ? Difference in the 

 nerves? How accounted for ? The rate of conduction along a nerve ? As compared with 

 electricity ? 



