ARE^ CENTRALES AND FOVE/E 



183 



local yellow pigmentation which creates a macula lutea and is a final 

 refinement in making the area centralis the spot of maximal visual acuity. 

 For the full comprehension of the meaning of the foveal depression we 

 must revert for a moment to the elements of physiological optics. 



A light ray passing through the cornea and lens and striking the 

 retina perpendicular to its surface will travel on through the retina with 

 its direction unchanged. It was long thought, however, that an appre- 

 ciable amount of the light would be absorbed and scattered in the retinal 

 tissue before reaching the visual-cell layer, thus not only being lost for 

 purposes of image-formation but, more important by far, tending to 

 blur the image. The depression of the fovea was then thought of as a 

 thin spot produced for the sake of thinning, and serving to remove tissue 



vitreous 



vitreous 



Fig. 76 — Local magnifying action of the foveal depression (based on the 

 central fovea of a hawk, Buteo b. borealis). 



from in front of the important central bouquet of cones in the area. This 

 theory must be discarded however, for in the best of areae (in lizards and 

 birds) the portion of the depressed retina, which is thinner than the 

 retina outside the area, is smaller than in arex with shallow foveas which 

 are known to be degenerate (Fig. 75). The retina, in life, is completely 

 clear and actually extinguishes no more light than the same thickness of 

 vitreous — which, of course, fills in the foveal excavation. 



A clue to the real meaning of the fovea (Fig. 76) was made available a 

 half-century ago in some observations of Valentin on the refractive index 

 of retinal tissue; but, it went unrecognized as a clue until very recently. 

 The data never seemed of any possible usefulness, and one finds no 

 figures given in modern reference books. But the index of the retina was 



