340 PHYSIOLOGY 



is sometimes laid down under the name of the ' Law of Forward 

 Direction.' It might be also spoken of as the irreciprocal conduction 

 of the nerve arc. The character of a reaction to any stimulus, applied 

 to the surface of the body, is determined by the course which the 

 impulse, excited in the afferent nerves, takes on entrance into the 

 central nervous system. This course is laid down by the connections 

 of the neurons through which the nerve impulse passes. In the 

 central nervous system therefore, more than in any other part of 

 the body, function is directly dependent on structure. Theoretically 

 if we had a perfect knowledge of the connections of the neurons in 

 the central nervous system and knew the nerve fibres affected by 

 any given stimulus, we should be able to prophesy exactly the result 

 of such stimulus. In the case of the simpler reactions this is already 

 possible, but in the higher parts of the nervous system the enormous 

 complexity of the systems of neurons excludes any possibility of 

 our forming more than a general idea as to the nerve paths traversed 

 in any given reaction ; and the variations which exist from 

 individual to individual must always prevent in the intact animal 

 an absolute prediction of the results of any stimulus. 



