302 THE HUMAN BODY. 



1. Ttie feeling of pain does not reside in the burned part 

 itself; for it is found that applying a hot object to the skin 

 or pinching it arouses no sensation if the nerves between 

 the skin and the nerve-centres be diseased or divided. 



2. The hot object when the nerves are intact originates 

 some change which, propagated along the nerves, excites a 

 condition of the nerve-centres accompanied by a feeling, in 

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

 fact that loss of sensation immediately follows division of 

 the nerves of the limb, but does not immediately follow the 

 injury of any of its other parts. The change propagated 

 along the nerve-trunks and causing them to excite the 

 nerve-centres is called a nervous impulse. 



3. When a nerve in the skin is excited it does not direct- 

 ly call forth muscular contractions; for if so, touching the 

 hot object would cause the limb to be moved even when 

 the nerve had been divided high up in the arm, while, as a 

 matter of observation and experiment, we find that no such 

 result follows if the nerve-fibres have been cut in any part 

 of their course from the excited, or, in physiological phrase, 

 the stimulated, part to the spinal marrow. It is therefore 

 through the nerve-centres that the nervous impulse trans- 

 mitted from the excited part of the skin is " reflected" or 

 sent hack to act upon the muscles. 



4. The preceding fact makes it probable that nerve-fibres 



How do we know that our feeling of pain does not reside in a 

 burned or pinched part of the skin? 



What does a touched hot object originate when the nerves are 

 healthy? What is a " nervous impulse"? 



Does a skin-nerve when excited produce directly a muscular 

 movement? Give reason for your answer. What happens to the 

 nervous impulse transmitted from the excited part of the skin? 



Is it probable that other nerve-fibres than those arising from the 

 are connectecj with the nerve-centres ? 



