NERVE 699 



reaction of degeneration after section of its motor nerve, only the 

 loss of excitability to induced currents is greater, and may even be 

 complete. The closing anodic contraction is stronger than the 

 closing kathodic the opposite of the ordinary law. The nerves 

 are inexcitable either to constant or induced currents. The reaction 

 of degeneration is only obtained from paralyzed muscles when the 

 paralyzing lesion is situated in the cells of the anterior horn from 

 which the motor nerves take origin, or below that level. Accord- 

 ingly, it is sometimes of use in localizing the position of a lesion. 

 For instance, a group of muscles might be paralyzed by a lesion in 

 the grey matter of the brain or in the nerve-fibres connecting this 

 with the grey matter of the anterior horn of the cord, or in the 

 grey matter of the anterior horn itself, or in the peripheral nerve- 

 fibres leading from this to the muscles. In the first two cases 

 the reaction of degeneration would be absent, although the muscles, 

 if the lesion was of long standing, would be atrophied to some 

 extent ; in the last two there would be acute atrophy of the muscles, 

 and the reaction of degeneration would be obtained. 



Trophic Nerves. There is no question that nerves exert 

 a very important influence upon the nutrition of the parts 

 supplied by them, in influencing the specific function of those 

 parts. So that in this sense all nerves are trophic nerves. The 

 fact that the proper nutrition of nerve-fibres and striated 

 muscular fibres is dependent on their connection with nerve- 

 cells has been by some writers generalized into the doctrine 

 that all tissues are provided with ' trophic ' nerves, which, 

 apart from any influence of functional activity, regulate the 

 nutrition of the organs they supply. But the evidence for this 

 view, when weighed in the balance, is found wanting ; and it may 

 be said that up to the present no unequivocal proof, experimental 

 or clinical, has ever been given of the existence of specific trophic 

 fibres, anatomically distinct from other efferent or afferent nerves. 



It is true that in various diseases and injuries of the nervous 

 system nutritive changes in the skin, and sometimes in the 

 bones and joint?, are apt to appear. But it is very difficult in 

 such cases to disentangle the effects produced by accidental 

 injuries acting on structures whose normal sensibility is lost or 

 lessened, or whose circulation is deranged, from true trophic 

 changes. The most that can be said is that there is some evidence 

 that the power of the skin to resist injury, and the capacity of 

 recovering from it, are diminished by interference with its nerve- 

 supply, so that a large sore may result from a trifling lesion, 

 and healing may be slow and difficult. Experimentally it has 

 been found that division of the trigeminus nerve within the skull 

 is sometimes followed by cloudiness of the cornea, going on to 

 ulceration, and ultimately inflammation and destruction of the 

 eyeball. Ulcers also form on the lips and on the mucous mem- 

 brane of the mouth and gums ; and the nasal mucous membrane 



