OF THE NERVES IN GENERAL. 497 



the centre the sensations produced by the impression of ex- 

 ternal agents. Cutting or tying them interrupts their func- 

 tions, and renders the parts below insensible and immoveable. 

 Irritation above the interruption produces sensations of pain 

 similar to what the irritation of the extremity of the nerve 

 would have produced. Irritation below the interruption pro- 

 duces contractions, like those which result from the irritation 

 of the origin of the nerve. 



785. It has been an object of research, since Herophilus 

 and Galen, whether there were not particular nerves for sen- 

 sation and others for motion. It was soon perceived that 

 there are sensorial nerves, as the first pair, the second, and the 

 auditory; motory nerves, as the third, the fourth, the sixth, the 

 hypoglossal, &c. ; and mixed nerves, as all the spinal nerves 

 which distribute themselves to the skin and muscles of the 

 trunk and members; and as the sub-occipital and trigemini. 

 But the paralyses and anestheses, which have been observed 

 sometimes united, and sometimes separate in the parts of the 

 body to which the nerves with double roots distribute them- 

 selves, led to the supposition that these nerves were composed 

 of distinct sensorial and motory filaments. The experiments 

 of Ch. Bell, those of Magendie, and my own, have clearly de- 

 monstrated that the posterior roots of the spinal nerves are 

 the sensorial, and the anterior the motory. 



786. The nerves are not entirely confined to the functions of 

 simple conductors: they have an activity of their own which 

 manifests itself when they are separated from the nervous cen- 

 tre; but this activity is much augmented by that of the spinal 

 marrow, as that of the spinal marrow is by the influence of the 

 encephalon;so that the cutting off of the encephalon diminishes 

 much the activity of the spinal marrow, and that of the spinal 

 marrow lessens much that of the nerves, and the nearer to a 

 muscle a nerve is cut off, the more the nervous influence upon 

 its contraction is diminished. 



787. Have the nerves a power of formation or regenera- 

 tion such, that on being cut across, their reunion shall have 

 the nervous texture and perform its functions? such even that 

 on being divided with loss of substance, they are reproduced? 



