310 
areae and one or more foveae. An 
area is a part of the retina in which rods 
are generally few or absent and cones 
are particularly tightly packed and 
individually represented by optic nerve 
fibers. Because of the corresponding 
increase in the number of nerve cells, 
the inner layers of the retina are 
thicker, so that a perceptible ridge or 
hummock is visible to ophthalmoscopic 
inspection. A fovea is a part of an 
area in which the density of cones is 
even higher, but the corresponding 
cells of the bipolar and ganglion layers 
are displaced radially from the center, 
leaving a pit. The pit may be a shal- 
ANNUAL REPORT SMITHSONIAN INSTITUTION, 1948 
It is tempting to correlate the first 
line of specialization with habits 
which give a special importance to 
the horizon, and, in fact, the birds 
illustrated are those of the wide open 
spaces—prairie, moor, sea, and lake 
—while there is no evidence thata 
ribbonlike area is ever found in the 
eyes of forest dwellers. The second 
type is characteristic of birds which, 
because they pursue fast-moving prey 
or, like the hummingbirds, feed from 
flowers on the wing, need an especially 
accurate perception of distance and 
relative speed. In these birds the 
central area is generally close to the 
visual-cell layer 
wcoor? 
retinol tissue 
oor 
SSO SECO SOG Sane" 
vitreous 
aM TAG cE 
visual-cell layer 
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retinal tissue 
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vitreous 
Ficure 4.—Diagram to show how refraction of light at the clivus of the fovea magnifies the 
image in the plane of the visual cell layer. 
(From Walls, 1943.) 
low, barely perceptible depression or 
a deep crater reaching down to almost 
the external limiting membrane (fig. 
5). 
The area must be regarded as poten- 
tially a region of high resolving power 
both for movement and static detail, as 
is the corresponding macular region 
of the human eye. It seems to be 
absent in many grain-feeding birds. 
When present it usually conforms to 
one or other of two distinct lines of 
specialization. In one line the area 
tends to extend like a ribbon round the 
retina in a horizontal plane. In the 
other, the tendency is toward the 
formation of distinct central and 
temporal areae (pl. 2). 
Based on central fovea of a buzzard, Buteo sp. 
optic axis (where aberration is least 
and the image consequently most 
perfect); the temporal area is so 
situated that the image of an object 
ahead falls on the temporal areae of 
both eyes simultaneously. 
The function of the avian fovea is 
still a problem. In man the neural 
layers of the retina are nourished by a 
network of blood vessels, through 
which light has to penetrate to reach 
the photosensitive layer. The radial 
displacement of the bipolar and gang- 
lion layers from the fovea permits 
light to reach the cones of this area 
unobstructed by a filter of blood capil- 
laries. Since the resolving power of 
foveal vision is very close to the 
