186 THE HUMAN BODY. 



CHAPTER XIII. 

 THE GENERAL PHYSIOLOGY OF THE NERVOUS SYSTEM. 



The Properties of the Nervous System. General Con- 

 siderations. If the finger of any one unexpectedly touches 

 a very hot object, pain is felt and the hand is suddenly 

 snatched away; that is to say, sensation is aroused and cer- 

 tain 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 incapable 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 circumstances the strongest effort of the 

 will of the individual is unable to bring about movement 

 of his hand. If, again, the nerves of the limb have uninjured 

 connection with the spinal cord, but parts of the latter, 

 higher up, between the brain and the point of junction of the 

 nerves of the brachial plexus with the cord, are injured, then 

 a sudden contact with the hot body will cause the arm 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. From the comparison of what happens in such 

 cases (which have been observed over and over again upon 

 wounded or diseased persons) with what occurs in the natural 

 condition of things, several important conclusions may be 

 arrived at: 



1. The feeling of pain does not reside in the burnt part it- 

 self; although that may be perfectly normal, no sensation 

 will be aroused by any external force acting upon it, if the 

 nervous cords uniting it with the centres be previously 

 divided. 



2. The hot body has originated some change which, when 

 propagated along the nerve-trunks, has excited a condition of 

 the nerve-centres which is accompanied by a sensation, in this 

 particular case a painful one. This is clear from the fact 

 that the loss of sensation immediately follows division of the 



