GENERAL PROPERTIES OF NERVES. 181 



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

 the nerve-centres ivhich 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 nerves of the limb, but not the injury of any of its 

 other parts; unless of such a character as to cut off the 

 supply of blood, when of course the nerves soon die, with 

 the rest. Even, however, some time after tying the vessels 

 which carry blood to a limb one can observe in experiments 

 upon the lower animals that sensibility is still retained if 

 the nerves be not directly injured. 



3. Wlien a nerve in the skin is excited by a burn or other- 

 wise it does not directly call forth muscular contractions; 

 for if so, touching the hot body would cause the limb to be 

 moved even when the nerve is divided high up in the arm, 

 and 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 burned .part to the 

 spinal marrow. It is therefore through the nerve-centres 

 that the change transmitted from the excited part of the 

 skin is reflected or sent back, to act upon the muscles. 



4. The last "deduction makes it probable that nerve-fibres 

 must pass from the centre to muscles as well as from the 

 skin to the centre. This is confirmed by the fact that if 

 the nerves of the limb be divided the will is unable to act 

 upon its muscles, showing that these are excited to con- 

 tract through the nerves. That the nerve-fibres concerned 

 in arousing sensation and muscular contractions are differ- 

 ent, is shown also by cases of disease in which the sensi- 

 bility of the limb is lost while the power of voluntarily 

 moving it remains, and by other cases in which the reverse 

 is seen, objects touching the hand being felt while it can- 

 not be moved by the will. We conclude then that cer- 

 tain nerve-fibres when stimulated convey something (a 

 nervous impulse) to the centres, and that these when ex- 

 cited may radiate impulses through other nerve-fibres to 

 distant parts, the centre serving as a connecting link be- 

 tween the fibres which carry impulses from without in, and 

 those which convey them from within out. 



