232 



EYE 



nerve fibres, with the addition of intercalated ganglionic cells, each 

 fibre ending either as a "cone" or as a "rod." Both, cones and 

 rods, look, however, outwards, i.e. 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, colourless. It consists of the following nine 

 layers : — 



1. Membrana limitans interna, an extremely thin colourless 



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 cseca, or blind spot, indi- 

 cated by a small funnel-shaped depression 

 in the middle 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. Liner 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 

 poAver 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 t\\Q form 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 fovese 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. 



Vertical Section through 

 THE Retina of a Sparrow. 

 Highly magnified. 



i.e. Membrana limitans exter- 

 na, perforated at its base by tlie 

 rods and cones ; L. i. Membrana 

 limitans interna ; P. Black pig- 

 ment on the top of the rods and 

 cones (3); 2, Layer of the fibres 

 of the optic nerve ; 3, 5, 7, Inner, 

 middle, and outer ganglionic 

 layers ; i, 6, Inner and outer 

 molecular layers. 



