182 ADAPTATIONS TO DIURNAL ACTIVITY 



consciousness, they are progressively eliminated and the area comes to 

 be a pure-cone island in a duplex sea of unmodified retina. 



As the rods are eliminated and the cones are aggregated and slen- 

 derized, the threshold of stimulation of the area tends to rise. The 

 areal cones would then go out of action, in failing illumination, before 

 the more massive extra- areal ones; but they counteract this tendency by 

 evolving longer and longer outer segments. This local thickening of the 

 visual-cell layer causes the external limiting membrane to bulge inward 

 toward the vitreous, and may even make the pigment epithelium bulge 

 outward against the chorioid ('fovea externa'). Retinal blood vessels, 

 where these are present (mammals) tend now to be excluded from the 

 area so as not to interfere with clear perception, and the chorioid may 

 have to thicken locally to carry the extra nutritional load. The increased 

 length of the visual cells has a fortuitous but very fortunate effect upon 

 the burden carried by the mechanism of accommodation (see pp. 30-1). 



a 



Fig. 75 — Well-developed (avian) and poorly-developed (human) iovex. x 271/2. 



Cross-hatched in each diagram is the portion of the foveal retina which is actually thinner 

 than the retina outside the area centralis. The superior avian fovea is less a 'thin spot' than 

 is that of man. a, foveal region of hawk, Buteo b. borealis. b, macular region of normal 

 human retina. 



The increase in the percentage of cones results in a great increase 

 in the number of bipolar and ganglion cells, since cones are summated 

 less in them to begin with, and less within the area than outside of it 

 — each cone, ideally, coming to have its own bipolar and ganglion cell 

 transmission-line to the brain. 



The thickenings of the visual-cell, outer nuclear, inner nuclear, gang- 

 lion-cell, and nerve-fiber layers add up to a local thickening of the retina 

 as a whole. Where this might become extreme, a fovea develops — not 

 to combat the thickening as such, but rather the convex surface thereof 

 which bulges into the vitreous. 



The Forea—The reader, stopping at any point in the above discussion, 

 would then have already read a complete description of some area cen- 

 tralis which actually exists in some vertebrate or other. Most arese do 

 not go on to develop a fovea, and fewer still of these have produced the 



