158 THE PHILOSOPHY OF BIOLOGY 



it was received, but what we make use of when we 

 utilise our experience is obviously the elements of that 

 experience, selected and re-integrated as we require 

 them. There must, then, be something like an analysis 

 of our perceptions, a dissociation of these into simple 

 constituents, and a means of restoring and recording 

 these constituents in such a way that they can be re- 

 combined in any order, and again made to enter into 

 our consciousness. 



It is quite possible to imagine such a mechanism. 

 Let us suppose that an efferent impulse enters the 

 cerebral cortex via any one axon : there is a perfect 

 labyrinth of paths along which the impulse may travel. 

 Everywhere in the central nervous system we come 

 upon interruptions of nervous paths formed by inter- 

 digitating arborescent formations. The twigs of these 

 arborescences do not, apparently, come into actual 

 contact with each other and the impulse leaps across 

 the gap between them. This gap is, of course, ex- 

 ceedingly narrow, and one can almost speak of it as a 

 membrane, since it must be occupied by some organised 

 substance. It has been called the synaptic membrane. 

 Let us suppose that a stimulus of a certain nature 

 passes through the synapse, modifying it physico- 

 chemically as it passes. Thereafter a stimulus of 

 similar nature will tend to pass across this particular 

 synapse, the resistance of the latter having been 

 decreased. It will thus tend to travel by a definite 

 tract through the central nervous system. Now the 

 latter we may regard in a kind of way as a very com- 

 plicated switchboard, the function of which is to place 

 any one stimulus (or series of stimuli) out of many in 

 connection with any one motor ^ mechanism (or series 



^ Or more generally effector mechanism. This enables us to include 

 reactions, such as secretory ones, which are not motor. 



