GENERAL PHYSIOLOGY OF THE NERVOUS SYSTEM. 187 



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. When 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 had been 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 trans- 

 mitted 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-fib? es 

 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 contract through 

 the nerves. That the nerve-fibres concerned in arousing 

 sensation and muscular contractions are different, is shown 

 also by cases of disease in which the sensibility 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 cannot be moved by the will. We con- 

 clude then that certain nerve-fibres when stimulated convey 

 something (a nervous impulse) to the centres, and that these 

 when excited 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. 



5. Further we conclude that the spinal cord can act as 

 an intermediary between the fibres carrying -in nervous im- 

 pulses and those carrying them out, but that sensations can- 

 not be aroused by impulses reaching the spinal cord only, 

 nor has the Will its seat there; volition and consciousness are 

 dependent upon states of the brain. This follows from the 

 unconscious movements of the limbs which follow stimula- 

 tion of its skin after such injury to the spinal cord as pre- 



