308 
Man’s ancestors lost this mechanism 
when they became nocturnal, and the 
human eye has only an unsatisfactory 
makeshift which becomes more and 
more inefficient throughout life. Both 
the muscles of accommodation and 
the muscles of the iris are unstriated 
and, compared with those of the bird, 
sluggish in action. 
The maximum accommodation of a 
20-year-old man is about 10 diopters 
(i. e., equivalent to inserting a specta- 
cle lens of 1/10-meter focal length in 
front of the eye). For the majority of 
cornea 
aqueous humour 
lens 
vitreous 
humour 
retina 
optic nerve 
ANNUAL REPORT SMITHSONIAN INSTITUTION, 1948 
in accommodation is enormous (fig. 
3, b). In the cormorants the iris is 
very muscular and the sphincter iridis 
muscle assists In compressing the an- 
terior part of the lens. 
The retina consists of four well- 
defined layers of cells, the outermost 
of which is the opaque, black, and 
nonreflecting chorioid layer. Imme- 
diately within this lies the layer of 
photoreceptive elements, the rods and 
cones. ‘hen comes the layer of bi- 
polar cells and, finally, the layer of 
ganglion cells, each of which is ex- 
scleral ossick 
pecten 
FicurE 2.—Lower halves of the right eye of man and left eye of swan. Position of image 
plane of human eye dotted. 
birds accommodation has a range of 
at least 20 diopters. For the cor- 
morant it is said to be 40-50 with the 
lens alone. Diving birds have to cope 
with the special difficulty that under 
water the outer surface of the cornea 
no longer contributes appreciably to 
the total refraction of the lens system; 
so that, in addition to accommodation 
for close range, the lens has to contrib- 
ute an additional 20 diopters or more 
to compensate for the loss of the cornea 
when the eye is immersed. As might 
be expected, Crampton’s muscle is 
degenerate in diving birds, while 
Briicke’s muscle is proportionately en- 
larged and the deformation of the lens 
The arrows point forward. 
tended into a nerve fiber which runs 
in the optic nerve to the brain (pl. 1). 
The names ‘rod’? and “‘cone” are 
descriptive of the shape of these cells 
in the human retina and are not very 
appropriate for birds, whose cones are 
much thinner and more rodlike than 
are human cones. There are, how- 
ever, important anatomical and physi- 
3 The outermost pigment-cell layer of the 
retina is distinct in its development from the 
innermost pigmented layer of the fibrous wall 
of the eyeball which is the chorioid, s. str., 
but structurally and functionally the pig- 
ment-cell layer belongs to the chorioid rather 
than the retina in birds and mammals, and it 
is convenient to use the term chorioid to 
include the pigment-cell layer. 
