476 NERVOUS SYSTEM 



the posterior preside over " the secret operations of the bodily frame, or 

 the connections which unite the parts of the body into a system." 



In 1822, Magendie, as the result of experiments on the exposed roots 

 in living dogs, stated that " he was able at that time to advance as 

 positive, that the anterior and the posterior roots of the nerves which 

 arise from the spinal cord have different functions, that the posterior 

 seem more particularly destined to sensibility, while the anterior seem 

 more specially connected with motion." 



All observations on the roots of the spinal nerves show that the 

 mixed nerves arising from the cord derive their motor properties from 

 the anterior roots and their sensory properties from the posterior roots : 

 and since the publication of an historical review of the question of 

 priority between Bell and Magendie (Flint, 1868), it has been ad- 

 mitted that the credit of this discovery belongs to Magendie. 



The anterior roots possess a certain degree of sensibility in addition 

 to their motor properties (Magendie). This sensibility, which is slight, 

 is derived from fibres from the posterior roots, which turn back to go to 

 the anterior roots. This has been positively demonstrated by the Wal- 

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

 the sensibility of the corresponding anterior root is lost, and degenerated 

 fibres appear, after a few days, in the anterior roots. This sensibility 

 of the anterior roots is called recurrent sensibility. Similar relations 

 exist 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, impulses, the nature of which is unknown, gener- 

 ated in the centres, are conducted from the centres to the peripheral 

 distribution of the nerves in the muscles, and are 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 interrupted, and motion in these 

 parts, in obedience to the natural stimulus, becomes impossible. While, 

 however, it is not always possible to induce generation 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 is divided, electric, mechanical or other stimulus 

 applied to the extremity connected with the centres produces no effect ; 

 but the same stimulus applied to the extremity connected with the 

 muscles is followed by contraction. The phenomena indicating that a 

 nerve retains its physiological properties are always manifested at its 

 peripheral distribution, and these do not vary essentially when the nerve 

 is stimulated at different points in its course. For example, stimulation 



