THE NERVOUS SYSTEM 65 



and in Ameiurus to occur in opposing pairs. Such 

 pairs with their restricted fields of action may serve as 

 the prototypes of the activating and the inhibiting 

 agents in central nervous operations. Hydrohumors, 

 on the other hand, with their powers of rapid and free 

 spread would exert broad and general influences on the 

 whole nervous organization. Such influences could 

 make themselves felt in the general tone of the central 

 nervous system, the kinds and degrees of personality, 

 and in those abnormal states that fill our hospitals. 



In this way neurohumors may play a very significant 

 part in nervous operations. Their occurrence as active 

 intermediaries in the chromatophoral system is a matter 

 of growing certainty. They also appear to have an im- 

 portant role in the excitation of smooth muscle and in 

 the control of the vertebrate heart-muscle. It is easy 

 to conceive of them as the effective agents in such cen- 

 tral nervous functions as activation and inhibition just 

 mentioned, in the relation of receptor cells to their 

 conducting neurones, and in that large body of nervous 

 interrelations where the integrity of nerve-units is de- 

 pendent upon the so-called trophic function. In these 

 numerous situations the idea of neurohumors affords 

 interesting hypothetical suggestions that lead at once to 

 experimental tests. The devising and applying of such 

 tests is no easy task, but it is just such exertions that 

 often yield the highest scientific returns. To approach 

 neurohumors and to understand their ways demands 

 the supreme effort of the investigator. 



