12 
examined microscopically by Slonaker a distinct visual area was 
present in all; no fovea was discovered in one; a single, round area 
or macular region was found in 59; two round arese (macular re¬ 
gions) in 11; while an additional band-like visual area was differ¬ 
entiated in 36. Seventy-two birds had a single, simple fovea, 11 had 
simple fovae, and 22 had a trough-like fovea. The single fovea, 
almost invariablv situated toward the nasal side of the animal, can 
generally be distinguished by means of the ophthalmoscope during 
life, but in birds with a double macula the second fovea, tempor¬ 
ally placed, will, I believe, be more difficult to locate. Slonaker 8 
found in most of the birds examined macroscopically after death a 
single fovea surrounded by a circular area—just as one sees them 
in man and the higher apes. In the goose and ring-neck plover he 
discovered a simple fovea in the center of a round macula, the lat¬ 
ter area extending horizontally across the retina. 
Two macular regions may be present, each with its fovea, joined 
by a short, band-like area, as in the sparrow-hawk, red-tailed buz¬ 
zard, and kingfisher. He found the most complex arrangement of 
the visual areas in the tern. Here the fovea temporalis is sur¬ 
rounded by a small, isolated, circular macular region and is not 
connected with the narrow, band-like area. The latter, however, 
stretches across the fundus and near its middle widens out to 
enclose the fovea nasalis in a larger circle than that enclosing the 
temporal fovea. 
So far, then, as concerns the shape and position of the areas 
of distinct vision in birds, a species may have one or two foveae and 
from one to three visual areas. 
The foveas vary in depth and position. In the owls, which possess 
binocular vision, and in birds which require keen vision, the fovea 
is deep and clearly cut. According to Slonaker, in those birds that 
possess two foveas, the fovea nasalis varies but little in position, 
while the locality of the temporal yellow spot depends largely upon 
the degree of divergence of the antero-posterior axes—the more 
divergence the greater the separation of the two foveae. The more 
the eyes look forward the more dependence is placed upon a single, 
deep, sharply-defined nasal, macular region and the shallower, less 
distinct and more merged in it become the temporal fovea and 
8. Loco, cit., page 458. 
