20 .Foster, Physical Basis of Psychical Events. 



These, then, are the fundamental principles of the 

 action of the nervous system on which I wish to insist as 

 a basis for any further considerations. The nervous 

 system is a congeries of chains of units, linked together in 

 definite ways. Each unit possesses individual powers and 

 characters, in part born with it, the outcome of events in 

 past times, in part developed during its own growth, as 

 determined by its own individual circumstances. The 

 several units act the one on the other, and the manifestations 

 of nervous life are brought out by the integration of the 

 differentiated activities of the individual units. Each unit 

 is essentially a unit acting as a whole, with all its parts 

 accordant. Any change of activity which occurs takes 

 place not within the unit while the phase of activity is 

 sweeping along the unit, but at the linkage where unit 

 joins on to unit. Further, the action of unit on unit is 

 varied ; it may be the inducement of that grosser kind of 

 activity, marked by an electric current of action which we 

 call a nervous impulse, it may be something else ; it may 

 be one of the many things of which we possess no clear 

 objective token, such as is supplied by the electric change, 

 and about which, therefore, we are as yet greatly in the 

 dark. 



Let me now turn to apply these general principles to 

 the problem of visual sensations. 



Put in its simplest form, what takes place in the 

 nervous mechanism of vision is somewhat as follows. 



Light falling on a cone (or rod, but for simplicity's sake 

 we may omit the double character of the initial elements) 

 sets up in this changes which are propagated along 

 the cone fibres. The cone and cone fibre are the respective 

 parts of an epithelial nervous unit or neurone, the cone 

 fibre being the axon. By the terminals of the axonic 

 cone fibre this first unit is linked on to a second unit, the 



