CHAPTER XX. 



THE GENERAL PHYSIOLOGY OF THE NERVOUS 

 SYSTEM. 



The Properties of the Nervous System. If one's finger 

 unexpectedly touches a very hot object, pain is felt arid the 

 hand is suddenly snatched away ; that is to say, sensation 

 is aroused and certain muscles are caused to contract. If, 

 however, the nerves passing from the arm to the spinal 

 cord have been divided, or if they have been rendered in- 

 capable of activity by disease, no such results follow. Pain 

 is not then felt on touching the hot body nor does any 

 movement of the limb occur ; even more, under such cir- 

 cumstances the strongest effort of the Will of the individual 

 is unable to cause any movement of his hand. If, again, 

 the nerves of the limb have connection with the spinal 

 cord, but parts of the cord are injured higher up, between 

 the brain and the point of junction of the nerves of the 

 arm with the cord, then contact with the hot object may 

 cause the hand to be snatched away, but no pain or other 

 sensation due to the contact will be felt, nor can the will 

 act upon the muscles of the arm, either to make them con- 

 tract or to prevent their contraction. From the compari- 

 son of what happens in such cases (which have been observed 

 again and again upon wounded or diseased persons), with 

 what occurs in the natural condition of things, several im- 

 portant conclusions may be reached: 



What usually results when a hot object is unexpectedly touched? 

 Under what circumstances do these results not occur? Can the Will 

 cause movement of the muscles of an arm whose nerves have been 

 cut? When the arm-nerves are intact but the spinal cord is injured 

 near the brain, what happens on touching a hot body? 



