232 
EVE 
nerve fibres, with the addition of intercalated ganglionic cells, each 
fibre ending either as a 
“ee 
cone” or as a “rod.” Both, cones and 
rods, look, however, outwards, 7.c. away from the inside of the eye, 
being inserted into the pigment layer of the chorioidea. 
The retina is transparent, devoid of blood-vessels, and, except 
certain cones, 
layers :— 
colourless. 
It consists of the following nine 
1. Membrana limitans interna, an extremely thin colourless 
VERTICAL SECTION 
THE RETINA OF A 
Highly magnified. 
THROUGH 
SPARROW. 
L.e. Membrana limitans exter- 
na, perforated at its base by the 
rods and cones; L.i. Membrana 
limitans interna; P. Black pig- 
ment on the top of the rods and 
cones (9); 2, Layer of the fibres 
of the optic nerve ; 3, 5, 7, Inner, 
middle; and outer ganglionic 
layers; 4, 6, Inner and outer 
molecular layers. 
membrane, which separates the retina from 
the corpus vitreum, and sends out fine 
radial fibres between the retinal fibres. 
2. Layer of the optic fibres ; they enter 
the eye at the fovea ceca, or blind spot, indi- 
cated by a small funnel-shaped depression 
in the middie of the optic nerve, whence 
they spread out at right angles; the fibres 
nearly lose their myelin sheaths, and con- 
sist chiefly of thin axial cylinders. 
3. Layer of inner ganglionic cells, inter- 
calated into the axial cylinders. 
4. Inner molecular or granular layer, 
a molecular substance, penetrated by the 
nerve fibres. 
5. Middle ganglionic layer, consisting 
of small cells which connect the 3rd with 
the 7th layer. 
6. Outer molecular or granular layer. 
7. Outer ganglionic layer, consisting of 
bipolar cells, continued through 
8. Membrana limitans externa, a colour- 
less membrane, into 
9. A layer of cones and rods; these 
elongated cylindrical bodies are the real 
light-perceiving end-organs of the optic 
nerve fibres. Into each of the bodies is 
intercalated a small lens-like body which possesses a higher refractory 
power than the surrounding parts. 
The cones contain drops of fat, 
mostly red or yellow in colour. 
Near the posterior pole of the eye, but somewhat excentrically 
placed, is the fovea centralis (the “ yellow spot” of Mammals), a slight 
depression in the retina, and composed chiefly of cones; it is the 
spot of most acute visuality. Many birds possess a second fovea 
more towards the outer or temporal side of the eye. One pair 
of these foveze seems to be used for monocular the other for binocular 
sight, so that the whole field of vision of birds possesses three 
points where vision is most acute. 
