CHAPTER LVII 



TROPHIC NERVES 



NERVES exercise a trophic or nutritive influence over the tissues and 

 organs they supply. Some nerves increase the huilding-up stage of 

 metabolism ; these are termed anabolic. Such a nerve is the vagus 

 in reference to the heart; when it is stimulated the heart beats 

 more slowly or may stop, and is thus enabled to rest and repair its 

 waste. The opposite kind of nerves (katabolic) are those which lead 

 to increase of work, and so increased wear and tear and formation of 

 waste products. Such a nerve in reference to the heart is the 

 sympathetic. 



There has been considerable diversity of opinion as to whether 

 trophic nerve-fibres are a distinct anatomical set of nerve-fibres, or 

 whether all nerves in addition to their other functions exercise a 

 trophic influence. 



When a nerve going to an organ is cut, the wasting of the nerve 

 itself beyond the cut constitutes what we have learnt to call 

 Wallerian degeneration, but the wasting process continues beyond 

 the nerve ; the muscles it supplies waste also, and waste much more 

 rapidly than can be explained by simple disuse. The same is seen 

 in the testicle after section of the spermatic cord ; and in the disease 

 of joints called Charcot's disease, the trophic changes are to be 

 explained by disease of the nerves supplying them. 



From these, and numerous other instances that might be given, 

 there is no question that nerves do exert a trophic influence; 

 the question, however, whether this is due to special nerve-fibres 

 has been chiefly worked out in connection with the fifth cranial 

 nerve. 



After the division of this nerve there is loss of sensation in the 

 corresponding side of the face: the cornea in two or three days 

 begins to get opaque, and this is followed by a slow inflammatory 

 process which may lead to a destruction not only of the cornea, but 

 of the whole eyeball. The same is seen in man ; when the fifth nerve 

 is diseased or pressed upon by a tumour beyond the Gasserian 



813 



