SENSE ORGANS. I21 



wanting — viz., where the optic nerve enters — and this 

 spot is blind; and (2) by the fact that the central spot or 

 fovea is the most sensitive spot in the retina, and there 

 the fibrous layer is entirely, and the middle layer almost 

 entirely, wanting. In this spot the bacillary layer is al- 

 most directly exposed to the impression of light. Thus, 

 then, the fovea is the most highly organized spot of 

 the retina. It differs from other parts in three particu- 

 lars : 1. The bacillary layer there consists only of cones. 



2. The cones there are much smaller than elsewhere. 



3. The bacillary layer is there almost directly exposed 

 to the influence of light. 



The distinctive functions of the rods and the cones 

 will come up for discussion hereafter. Suffice it to say 

 now that the perception of color seems to reside in the 

 cones alone. 



Visual Purple. — There has recently been found in 

 the outer or terminal ends of the rods, but not the cones, 

 a purplish red substance, which probably has an important 

 but imperfectly understood function in vision, and is 

 therefore called visual purple. It is bleached by light, 

 and again restored by darkness. Photographic images 

 (optograms) of objects may be taken on the purple retina 

 and by appropriate means may be fixed.* The discov- 

 ery of this substance naturally excited hopes that its 

 study would solve the mystery of sensation by reducing 

 it to a chemical process ; but these hopes have not been 

 realized, for it is now known that the visual purple is 

 not present in all animals, nor does it exist in the cones, 

 and therefore is not present in the fovea, which is, nev- 

 ertheless, the most sensitive spot in the retina both to 

 form and color, though not to simple faint light. The 

 visual purple, therefore, is certainly not essential to the 

 perception of either light or color. 



* Foster's Physiology, p. 1254. 



