SENSE ORGANS OF 
ological differences between rods and 
cones which seem to be consistent in 
all vertebrates. Each cone typically 
connects with its own group of bipolar 
cells and each such group with its own 
ganglion cell. It is rare, therefore, to 
find that an optic nerve fiber repre- 
sents more than four or five cones, and 
in the area and fovea (see below) there 
is strict one-to-one correspondence. 
On the other hand there are always 
many rods connected with a rod bi- 
tenacular ligament, 
Crampton's m. 
scleralossicle 
(a) 
Bruckes m. 
ciliary 
309 
suited for the registration of detail in 
a brightly illuminated visual field. 
The rod system, because of its exten- 
sive summation, is comparatively in- 
capable of resolving detail but can 
register an achromatic picture of the 
gross features of the field at ilumina- 
tions so low that the cone system is 
quite blind. We may expect, there- 
fore, to find, as in fact we do, that 
cones are predominant in the retinae 
of eyes which are exclusively diurnal 
BIRDS—PUMPHREY 
[ lens 
zonule 
body 
FicurE 3.—a, the mechanism of accommodation (orig.); 4, the lens of the cormorant’s eye 
at rest (full line) and fully accommodated (dotted). 
polar and often many rod bipolars 
connected with a single ganglion cell. 
An optic nerve fiber may therefore 
represent anything from 20 to several 
thousand rods. 
Rods will respond to an intensity of 
illumination from 1 to 10 thousand 
times less than is needed to excite the 
cones. But within their more re- 
stricted range the cones are responsive 
to much smaller percentage changes 
in intensity; and it is the cones which 
are responsible for color vision. 
The cone system is consequently 
(From Franz, after Hess, 1910.) 
and rods in eyes used only at night. 
Acuity (the power of resolving detail) 
and sensitivity are inverse require- 
ments which cannot both be fully satis- 
fied in the same eye. And in man a 
compromise is reached by cone pre- 
dominance near the axis of the eye 
where the image is coincident with the 
retina and rod predominance toward 
the periphery. 
The retina of a diurnal bird’s eye is 
cone-rich everywhere, but there is 
frequently a good deal of local differ- 
entiation. There may be one or two 
