NEUKO-MUSCULAR MECHANISM 117 



has been already considered (p. 15), and it has been seen that 

 it may be either general or unilateral, producing the pheno- 

 mena of positive and negative phototaxis. 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 such 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. 



But 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 coming 

 from one point will stimulate one set of cells which will excite 

 one part of the brain, and light from another will act upon 

 other cells which will excite another part of the brain, and 

 thus not merely the degree of illumination but the source of 

 illumination becomes distinguishable. 



It is by this arrangement that it becomes possible to form 

 ideas 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 degrees 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 degree, another set of cells to another degree, and a 

 third set of cells to a third degree ; and the different stimula- 

 tion of these different sets of cells leads to a different excita- 

 tion of separate sets of neurons in the brain. These changes in 

 the brain are accompanied by the perception of the three parts 

 differently illuminated. From the previous training of the 

 nervous system we are taught to interpret this as due to a 

 corner. But this interpretation is simply a judgment based 

 upon the sensations, and it may or may not be right, and 

 instead of actually looking at a corner we may be looking at 

 the picture of one. 



From the very first it must be remembered that the modifica- 

 tion of our consciousness which we call vision is not directly due 

 to external conditions, but is a result of changes set up in the 



