208 COMPARATIVE PHYSIOLOGY OF THE BRAIN 



contained in no other organ of the body, namely, io- 

 dine. It is thus conceivable that hereditary mental 

 diseases are chemical diseases. The germ-cells may 

 in these diseases also be influenced by the poisons 

 circulating in the blood. 



4. If we thus deny the immediate influence of the 

 central nervous system on the germ, and assume a 

 chemical theory of heredity, it might still be possible 

 that the central nervous system could influence hered- 

 ity indirectly, in so far as it can affect the chemical 

 processes of the body. As illustrations of a chemical 

 effect of the nerves, the fact is mentioned that stimu- 

 lation of the nerves of certain glands produces a secre- 

 tion. Mathews has shown, however, that in cases 

 where stimulation of the sympathetic produces a secre- 

 tion, the glands contain muscular fibres which contract 

 when stimulated, and in this way press a liquid out of 

 the ducts (4). (Conditions seem to be different in the 

 case of the secretion produced by stimulation of the 

 chorda, but it is also possible that in this case the secre- 

 tion is only an indirect effect of the stimulation caused 

 by changes in the circulation.) There are other 

 cases of an apparent chemical effect of the nerves. 

 The fact that herpes zoster follows the nerves has 

 led many to assume that this disease is caused by a 

 trophical influence of the nerves. But we know that 

 in the case of rabies the micro-organism or the poison 

 creeps along the nerves. Goltz has found that ulcer- 

 ations and suppurations occur on the skin behind the 

 cut after division of the spinal cord, which are so sym- 



