184 ADAPTATIONS TO DIURNAL ACTIVITY 



carefully measured by Valentin in a number of animals and was found 

 to be always substantially higher than that of the vitreous. What this 

 means is that if a light ray should strike the vitreoretinal boundary at 

 anything but a right angle it will be refracted away from an imaginary 

 perpendicular to the surface at the point of its impact. 



The foveal depression is designed deliberately to take advantage of 

 this refraction. The foveal portion of the retinal image is expanded on 

 its way through the retinal tissue, and is thus magnified somewhat when 

 it reaches the level of the visual cells. In birds the magnification is about 

 13% linearly, 30% in area; and it is probably greater in lizards. The 

 linear increase directly affects visual acuity. The areal increase improves 

 the perception of 'pattern', though it adversely affects sensitivity to 

 external illumination. A part of the lengthening of foveal cones, two 

 advantages of which have already been mentioned, is perhaps in com- 

 pensation for the local dimming of the expanded portion of the image. 



When an area centralis has done everything else possible to increase 

 the number of receptor-units over which the image will fall, the further 

 increase afforded by a deep fovea makes the production of one decidedly 

 worthwhile — nay, mandatory, for the convex bulge in the internal limit- 

 ing membrane over a highly-developed area centralis would tend to 

 converge the rays of light and make the image, at the level of the visual- 

 cell layer, smaller. The shallow depression in the area centralis of a soft- 

 shelled turtle (Fig. 78b) or the average teleostean fovea probably does 

 little more than cancel the minifying effect of the area's convex inner 

 surface. The deeper the actual depression goes below the original level 

 of the retina, the higher the mound or 'circumfoveal eminence' created 

 around the depression by the displaced tissue. Since a continuous steep 

 slope is thus produced from the crest of the mound to the bottom of the 

 depression, this sloping surface becomes an effective magnifying device, 

 of optically unique description. 



Distribution — No lamprey has an area centralis, but one occurs in 

 Mustelus — the only genus of sharks known for certain to have any 

 cones at all. It is marked by a noticeable concentration of ganglion 

 cells (Fig. 77a). An area centralis is very commonly seen in bony fishes, 

 and a fovea (Fig. 77b) has been found in a score or so of teleosts (see 

 Table III, p. 187) , never as deep as in lizards but with both rods and twin 

 cones excluded from it. The areae centrales of frogs, most turtles (Fig. 

 78), and all crocodilians are devoid of foveae and are imperfect in that 

 they contain rods as well as cones— indeed, the crocodilian is nocturnal 



