472 TROPHIC NERVES. [BOOK n. 



mechanisms of the eyelids and tears, it seems reasonable to 

 suppose that the inflammation in question might simply be the 

 result of the irritation caused by dust and contact with foreign 

 bodies, to which the eye, no longer guided and protected by 

 sensations, these being destroyed by the section of the nerve, 

 became subject. In the same way the ulcers on the lips and 

 gums might be explained as injuries inflicted by the teeth on 

 those structures in their insensitive condition. And some ob- 

 servers maintain that the inflammation of the eye may be greatly 

 lessened or altogether prevented if the organ be carefully covered 

 np and in all possible ways protected from the irritating influences 

 of foreign bodies. Other observers however have failed to prevent 

 the inflammation in spite of every care. This negative result is in 

 itself no strong argument, but the question cannot yet be con- 

 sidered as entirely cleared up. 



In a mammal division of both vagi is followed by pneumonia 

 (inflammation of the lungs) ending in death. This has been ad- 

 duced as an instance of the trophic action on the pulmonary tissues 

 of certain fibres of the vagi ; but the real explanation seems to be 

 that, owing to a paralysis of the oesophagus and larynx caused by 

 section of the vagi, food accumulating in the pharynx passes into 

 the air-passages and so sets up the pneumonia. In birds death 

 follows, sometimes from pneumonia of a similar causation, but more 

 frequently from inanition on account of the food not being able to 

 enter the stomach. The immediate cause of death however appears 

 in many cases at all events, both in birds and mammals, to be a 

 paralysis of the heart, and the histological changes (acute fatty 

 degeneration) observable in the cardiac muscles are of such a 

 character as to suggest a trophic action of the vagus fibres on that 

 tissue. 



Other instances of nerves manifesting even a doubtful trophic 

 action as the result of experimental interference are rare; yet there 

 seems to be no reason why the fifth nerve or the vagus should be 

 conspicuous in possessing trophic fibres. When the sciatic nerve of 

 the frog is divided, no nutritive alterations beyond those explicable 

 as the result of loss of function are observed ; and indeed the ma- 

 jority of the effects on growth and nutrition resulting from the 

 section of nerves, or from paralysis, can be referred to the absence 

 of the usual functional activity, accompanied in some cases with an 

 altered vascular supply. It must be remembered however that 

 functional activity is itself the result of metabolic and therefore 

 nutritional changes ; and in cases of inhibition, as for instance in 

 the action of the vagus on the heart, we seem to have illustrations 

 of a nerve producing metabolic changes leading not to the exercise 

 but to the arrest of functional activity. 



Taking all things into consideration, we may venture to say 

 that the numerous phenomena of disease, joined to the facts men- 



