2io COMPARATIVE PHYSIOLOGY OF THE BRAIN 



mation. The latter might be the case if Gaule's 

 statement is correct, namely, that histological changes 

 can be shown in the cornea ten minutes after 

 the division of the trigeminus (6, 7). In this 

 case it can only be that the power of resistance 

 or, more accurately speaking, the chemical nature 

 of the tissue is changed as a result of the lesion 

 of the nerve. If this be true, it does not force 

 us to the assumption of specific trophic nerves ; if 

 it is true that the influence of every nervous im- 

 pulse on the affected tissue is chemical, all nerves are 

 in one sense trophic, and it would be quite erroneous 

 to maintain that certain nerves serve trophic func- 

 tions exclusively while others are sensory and motor. 

 There are no specifically trophic nerves, but it is pos- 

 sible that many nerves produce indirectly (for instance, 

 through disturbances of the circulation and limitation 

 of the supply of oxygen) such extensive chemical 

 changes that morphological changes of the tissue 

 ensue. 



If this is in reality the case, a possibility still exists 

 that the central nervous system also affects the sex- 

 ual cells indirectly, in so far as disturbances of circu- 

 lation and hence chemical changes are produced, which 

 may modify the sexual cells contained in the testes 

 and ovaries chemically. Thus there might be a very 

 remote chance that brain-activity of one generation 

 might lead to the formation of chemical substances 

 which affect the sexual cells. It is difficult to under- 

 stand, however, what should cause these sexual cells 



