MOTOR AND SENSORY NERVES. 523 



The anterior roots possess a certain degree of sensibility in addition to 

 their motor properties (Magendie). This sensibilitity, which is slight, is de- 

 rived from fibres from the posterior roots, which turn back to go to the an- 

 terior roots. This fact has been positively demonstrated by the Wallerian 

 method. When a posterior root is divided beyond the ganglion, the sensi- 

 bility of the corresponding anterior root is lost, and degenerated fibres appear, 

 after a few days, in the anterior roots (Schiff). This sensibility of the an- 

 terior roots is called recurrent sensibility. Similar relations are observed 

 between certain of the motor and sensory cranial nerves. 



Mode of Action of the Motor Nerves. As regards the normal action of 

 the motor nerves, a force, the nature of which is unknown, generated in the 

 centres, is conducted from the centres to the peripheral distribution of the 

 nerves in the muscles, and is here manifested by contraction. Their mode 

 of action, therefore, is centrifugal. When these motor filaments are divided, 

 the connection between the parts animated by them and the centre is inter- 

 rupted, and motion in these parts, in obedience to the natural stimulus, be- 

 comes impossible. While, however, it is not always possible to induce gen- 

 eration of nerve-force in the centres by the direct application of any agent to 

 them, this force may be imitated by stimulation applied to the nerve itself. 

 A nerve that will thus respond to direct stimulation is said to be excitable. 



If a motor nerve be divided, electric, mechanical, or other stimulus ap- 

 plied to the extremity connected with the centres produces no effect ; but the 

 same stimulus applied to the extremity connected with the muscles is fol- 

 lowed by contraction. The phenomena indicating that a nerve retains its 

 physiological properties are always manifested at its peripheral distribution, 

 and these do not essentially vary when the nerve is stimulated at different 

 points in its course. For example, stimulation of the anterior roots near the 

 cord produces contraction in those muscles to which the fibres of these roots 

 are distributed ; but the same effect follows stimulation of the nerve going to 

 these muscles, in any part of its course. 



As far as their physiological action is concerned, the individual nerve- 

 fibres are entirely independent ; and the relations which they bear to each 

 other in nervous fasciculi and in the so-called anastomoses of nerves involve 

 simple contiguity. Comparing the nerve-force to galvanism, each individual 

 fibre seems completely insulated ; and a stimulus conducted by it to muscles 

 never extends to the adjacent fibres. That it is the axis-cylinder which 

 conducts and the medullary tube which insulates, it is impossible to say with 

 positiveness ; but it is more than probable that the axis-cylinder is the only 

 conducting element. 



The generation of a motor impulse may be induced by an impression 

 made upon sensory nerves and conveyed by them to the centres. If, for 

 example, a certain portion of the central nervous system, as the spinal cord, 

 be isolated, leaving its connections with the motor and sensory nerves 

 intact, these phenomena may be readily observed. An impression made 

 upon the sensory nerves will be conveyed to the gray matter of the cord 

 and will induce the generation of a motor impulse by the cells of this part, 



