FACTS 189 



tional assimilation, the route followed by the influx several 

 times in succession is more clearly traced in the nerve centres 

 of the animal and that, under the same conditions, the 

 same stimulus will determine the same activity of the animal 

 as a whole. 



A fisher with hook and line remarks that the flight of 

 certain insects taken in a certain way along the river's 

 surface makes the trout leap and snap at them ; and so he 

 will imitate as closely as possible with his artificial fly the 

 motion of the natural one. If he succeeds, the trout will 

 leap and swallow the hook. But for this it is necessary 

 that the phenomenon should not be mixed up with other 

 factors of action, such as sound vibrations for example, 

 when the fisher makes a noise otherwise the observer 

 will be unable to foresee the result of the experiment. He 

 cannot tell beforehand whether the tendency to flight deter- 

 mined by the perception of sound will not prevail over 

 the appetite determined by the bait. 



Trophic Role of Nerves 



In animals with highly differentiated tissues, we have 

 seen that functional assimilation takes a particular form. 

 A muscle which contracts and is already as much muscle 

 as it can be, does not become more muscle ; but it does 

 grow as muscle by functional assimilation, when the con- 

 traction is really the function of muscle. 



But we now see that what determines the muscle to 

 contract or the gland to secrete, is the nerve influx arriving 

 and determining a rupture of its colloid equilibrium. We 

 are therefore brought to this interesting conclusion at the 

 same time that the nerve determines the functioning of a 

 cellular element, its role with regard to the element is 

 trophic or nutritive. 



