2 Foster, Physical Basis of Psychical Events. 



And I am given to understand that I am to speak not 

 so much to those who already know as to those who 

 desire to learn. 



I will take as a sort of text the physical events which 

 lie at the bottom of that element of psychical life which 

 we call a visual sensation. When 'light falls upon' the eye 

 it produces in us, under ordinary circumstances, a psychical 

 effect, a visual sensation. In order that the effect may be 

 produced, the integrity of certain nervous structures is 

 necessary. The retina within the eye is connected by 

 the optic nerve and optic tract with the three nervous 

 bodies called the corpus geniculatum, the corpus 

 quadrigeminum and the pulvinar or hind part of the 

 optic thalamus. Each of these three bodies is in turn 

 connected by nervous strands with the superficial grey 

 matter or cortex of the cerebral hemisphere in the 

 occipital region. These nervous structures, the retina, the 

 optic nerve and tract, the three bodies just spoken of, 

 which I may call the intermediate bodies, and the 

 occipital cortex constitute together a nervous mechanism, 

 a visual mechanism, the integrity of which is essential 

 to the development of visual sensations. Destruction 

 of or interference with any part of this mechanism at 

 once does away with or modifies the development of visual 

 sensations, brings about blindness or defective sight ; 

 whereas on the other hand destruction of or interference 

 with any other part of the nervous system does not 

 necessarily and directly do away with or interfere with the 

 mere development of visual sensations, though it may, in 

 one way or another, indirectly affect vision. Obviously, 

 when * we see ', the vibrations of the ether impinging on 

 the retina give rise in this mechanism to events, the 

 issue of which is a visual sensation. The visual 

 sensation is a psychical thing. What are the physical 

 changes, the changes which we can objectively lay hold 



