778 NERVE 



horn from which the motor nerves take origin, or below that level. 

 Accordingly, 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 ttye 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 joints, 

 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 ulti- 

 mately inflammation and destruction of the eyeball. Ulcers also form 

 on the lips and on the mucous membrane of the mouth and gums ; and 

 the nasal mucous membrane on the side corresponding to the divided 

 nerve becomes inflamed . But in this case the sensibility of the eye is 

 lost, and reflex closure of the eyelids ceases to prevent the entrance of 

 foreign bodies. The animal is no longer aware of the contact of particles 

 of dust or bits of straw or accumulated secretion with the conjunctiva, 

 and makes no effort to remove them. The lips, being also without 

 sensation, are hurt by the teeth, particularly as the muscles of mastica- 

 tion on the side of the divided nerve are paralyzed, and decomposed 

 food, collecting in the mouth, and inhaled dust in the nose, will tend 

 still further to irritate the mucous membranes. There is thus no more 

 need to assume the loss of unknown trophic influences in order to 

 explain the occurrence of the ulcerative changes than there is to 

 explain the production of ordinary bed-sores, bunions or corns on parts 

 peculiarly liable to pressure. And, as a matter of fact, if the eye be 



