MOTOR AND SENSORY NERVES 475 



The sensibility of the sensory nerves disappears from the periphery 

 to the centres, as is shown in dying animals and in experiments with 

 anesthetics. The sensibility is lost, first in the terminal branches of 

 the nerves, next in the trunks and in the posterior roots of the spinal 

 nerves, and so on to the centres. 



Nerves that have been divided may be regenerated if anatomical 

 union of the divided ends can be secured ; and this sometimes takes 

 place several months after injury to the nerves, the regeneration occur- 

 ring by the formation of new fibres. Mixed nerves are regenerated in 

 this way, and conduction in both directions finally is restored. The 

 sensory conduction appears first, and next, the conduction of motor 

 impulses. The restoration of the physiological 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 can be united to the 

 divided end of a sensory nerve. 



MOTOR AND SENSORY NERVES 



Aside from nerves possessing special properties, such as 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 (i) centrifugal, or 

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

 conduct impulses from the centres to the muscles and excite muscular 

 contraction. The sensory 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 physiological 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-spinal 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 has been 

 quite generally attributed to Charles Bell. 



The first definite statement in 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 



