NERVE 137 



capable of setting up chemical changes, as in photography, 

 do not produce visual sensations. 



1. The action of light upon the protoplasm of lower 

 organisms has been already considered (p. 26), and it has 

 been seen that it may be either general or unilateral, pro- 

 ducing the phenomena of positive or negative phototaxis. 



2. In more complex animals, special sets of cells are set 

 apart to be acted on by light, and these are generally 

 imbedded in pigmented cells to prevent the passage of light 

 through the protoplasm. Such an accumulation of cells 

 constitutes an eye, and, in the simpler organisms, an eye can 

 have no further function than to enable the presence or 

 absence of light or various degrees of illumination to produce 

 their effects through the impulses which are sent to the 

 nervous system. 



3. In the higher animals these cells are so arranged that 

 certain of them are stimulated by light coming in one 

 direction, others are stimulated by light coming in another, 

 and while the former are connected with one set of synapses 

 in the brain, the latter are connected with another. Thus, 

 light from one point will stimulate one set of cells which, 

 through the nerve fibres passing to the central nervous 

 system, will excite one part of the brain, and light from 

 another point will act upon other cells which will excite 

 another part of the brain, and thus not merely the degree of 

 illumination but also the source of illumination becomes 

 distinguishable. 



By this arrangement it becomes possible to gain 

 knowledge of the shape of external objects. One directs the 

 eye to the corner of the ceiling, and the idea that it is a 

 corner is due to the fact that three different decrees of 

 illumination are appreciated, and that these can be 

 localised — one above, one to the right, and one to the left. 

 One set of cells is stimulated to one desrree. another set of 

 cells to another degree, and a third set of cells to a third 

 degree ; and the different stimulation of these different sets 

 of cells leads to a different excitation of separate sets of 

 neurons in the brain. These changes in the brain are 

 accompanied by the perception of the three parts differently 



