522 NERVOUS SYSTEM. 



Nerves that have been divided may be regenerated if anatomical union of 

 the divided ends can be obtained ; and this sometimes takes place several 

 months after injury to the nerves, the regeneration occurring by the forma- 

 tion of new fibres. Mixed nerves are regenerated in this way, and conduction 

 is finally restored in both directions. The sensory conduction appears first, 

 and next, the conduction of motor impulses. The restoration of the physio- 

 logical properties of the nerves occupies several weeks. The central end of a 

 mixed nerve has been made to unite with the peripheral end of another 

 mixed nerve, but it is doubtful whether a divided end of a motor nerve is 

 ever united to the divided end of a sensory nerve. Experiments upon this 

 latter point are not entirely satisfactory. 



MOTOR AND SENSORY NERVES. 



Aside from the nerves possessing special properties, such as the nerves of 

 sight, hearing, smell, taste and, according to some physiologists, nerves of 

 touch, temperature, sense of weight and muscular sense, the cerebro-spinal 

 nerves present two kinds of fibres. These are (1) centrifugal, or motor 

 fibres, and (2) centripetal, or sensory fibres. The motor fibres conduct im- 

 pulses from the centres to the muscles and excite muscular action. The sen- 

 sory fibres conduct impressions from the periphery to the centres, which are 

 appreciated either as ordinary sensation or as pain. As regards the nerves 

 arising by two roots from the spinal cord, the exact anatomical and physio- 

 logical divisions into motor and sensory were first made by Magendie, in 

 1822. As will be seen farther on, this division is distinct for the cranial 

 nerves, so that it is universal in the cerebro-spiual system. The importance 

 of the discovery of the distinct properties of the two roots of the spinal 

 nerves is such that it merits at least a brief historical account, particularly 

 as this discovery is quite generally attributed to Charles Bell. 



The first definite statement with regard to distinct properties of the two 

 roots of the spinal nerves was made by Alexander Walker, in 1809, who said 

 that the posterior roots were for motion and the anterior roots for sensation, 

 the exact reverse of the truth. 



In a pamphlet privately printed by Charles Bell, probably in 1811, and 

 " submitted for the observations of his friends," the view was advanced that 

 the anterior roots are both motor and sensory and that 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 upon 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 particu- 

 larly destined to sensibility, while the anterior seem more specially con- 

 nected with motion." 



It is now universally admitted that the mixed nerves arising from the 

 spinal cord derive their motor properties from the anterior roots and their 

 sensory properties from the posterior roots. 



