526 NERVOUS SYSTEM. 



however, the sense of the intermediate part is lost, the feeling in the hand or 

 foot, as the case may be, remaining as distinct as ever, the impression being 

 that the limb is gradually becoming shorter. It was noted by Gueniot, that 

 the sense of the limb becoming shorter exists in about half of the cases of am- 

 putation in which cicatrization goes on regularly ; and in these cases, the pa- 

 tient finally experiences a feeling as though the hand or foot were in direct 

 contact with the stump, 



Physiological Differences between Motor and Sensory Nerve-Fibres. It 

 has not been shown that there is any essential anatomical difference between 

 the conducting elements of motor and sensory nerve-fibres ; but the physio- 

 logical differences are sufficiently distinct, as has already been seen. Under 

 normal conditions, motor fibres conduct motor impulses in but one direction, 

 and these fibres are insensible. Sensory fibres conduct impressions always in 

 the opposite direction, and they do not conduct motor impulses. Certain 

 experiments, however, have led some physiologists to adopt the view that the 

 conducting properties of the nerves themselves, both motor and sensory, are 

 identical, and that the direction of conduction depends upon the kind of 

 centres with which nerves are connected. These experiments are the fol- 

 lowing : 



It is said that the peripheral end of a divided motor nerve, the sublin- 

 gual, can be made to unite with the central end of a sensory nerve, the 

 lingual branch of the fifth ; and that after a time motor impulses are con- 

 ducted by the sensory fibres and sensory impressions, by the motor fibres. A 

 careful study of these experiments, however, shows that the results are far 

 from satisfactory. 



Another experiment is grafting the end of the tail of a rat into the 

 skin of the back (Bert). When the union has become complete, the tail is 

 divided at its root and the sensory conduction, after five or six months, takes 

 place in a direction opposite to the normal. While this experiment may be 

 regarded as showing that sensory fibres may be made to assume such rela- 

 tions with other sensory fibres as to change, after a time, the direction of 

 conduction, it has no absolutely direct bearing upon the question of the 

 physiological identity of motor or sensory fibres. 



The experiments just mentioned seldom succeed, and the results of 

 union of motor with sensory nerves are quite indefinite ; but the divided 

 ends of mixed nerves readily reunite, and it is not difficult to establish a 

 union between the central and peripheral ends of two different mixed 

 nerves. It is hardly reasonable to assume that in these instances, each and 

 every divided end of a motor fibre selects another motor fibre with which it 

 unites, and that the same occurs with sensory fibres ; but it would seem that 

 in a divided mixed nerve, a certain number of fibres of each kind must 

 unite with certain fibres that have similar physiological properties. Com- 

 plete physiological regeneration of divided nerves is always slow, and fre- 

 quently the regeneration never becomes complete. The fact, also, that 

 curare destroys the physiological properties of motor nerves, leaving the sen- 

 sory nerves intact, has a very important bearing upon the question under 



