99 



bility of centripetal nerve-fibres in the five following different 

 parts of their length. 



1st. The part contained in the skin. 



2d. The part of a nerve extending from the skin to the spinal 

 cord. 



3d. The posterior roots of the spinal nerves. 



4th. The part of the posterior roots attached to the spinal 

 cord. 



5th. The part of the cutaneous nerve-fibres contained in the 

 gray matter of the spinal cord. 



The fibres existing in the gray matter of the spinal cord ap- 

 pear to be inexcitable, at least by our ordinary means of excita- 

 tion. Of the four other parts, the less excitable is the nerve be- 

 tween the ganglion and the skin. The excitability of the pos- 

 terior roots is less than that of the skin and that of their part 

 attached to the spinal cord. Of these two last parts the skin is 

 less excitable than the other. 



I measured the excitability by the degree of pain or of reflex 

 action. The differences are much more easily found for the re- 

 flex action than for the pain. 



Is it because they have been connected with the cells of the 

 central gray matter of the spinal cord, that the centripetal fibres, 

 contained in that gray matter, are not excitable ? If it is so, 

 there is a difference between these cells and those of the gan- 

 glions on the posterior roots, because the connection of these fibres 

 with the cells of these ganglions does not prevent their being 

 excitable.* 



From the facts above related I conclude that the same nerve- 

 fibre, in different parts of its length, may have very different de- 

 grees of excitability. 



XXXI. THE AUDITIVE NERVE IS A NERVOUS CENTRE. 



In an anatomical point of view there is no doubt that the 

 auditive nerve is a nervous centre. This is proved by the fact 

 that cells of gray matter are found, not only in the terminal part 

 of the nerve, but also in its trunk, in many animals, according 

 to the researches of Stannius, Corti, Kb'lliker, and myself. 



In a physiological point of view, the fact I have discovered, 

 (see Art. V. p. 21,) viz., that any injury to the acoustic nerve 



