236 NUNNELEY, ON THE RETINA. 
cleated cerebral cells forming the fourth layer of the retina, 
but perfectly like those in the optic nerve, may be found. 
In the retina the fibres are somewhat wavyeas they pass 
forwards, they reflect the light strongly, and at times, though, 
I think, not very frequently, they ferm anastomoses with each 
other; or at any rate appear to do so, at long intervals, and 
at acute angles; and my impression is, they terminate in the 
granular layer, though this is by no means demonstrated. 
In the fish these fibres are larger than in mammalia, not 
so distinctly tubular, run straighter, are not seen to moscu- 
late, and do not, so far as I have observed, so readily become 
varicose, though they sometimes do, as in in the cod (PI. XI, 
fig. 10), altogether presenting more the character of gray 
fibres, without the white tissue, than in higher animals. The 
optic nerve projects further into the eye of fish than it does 
inthem. As the retinal fibres appear to me to be identical in 
structure with the nerve-fibres, so I believe them to be in 
function, and to merely convey the impressions to the senso- 
rium which the true retinal elements perceive. ‘This would 
satisfactorily explain why the part of the eyeball where the 
nerve enters it, and where only nerve-fibres exist, must neces- 
sarily be insensible to visual impressions. 
6. Hyaloidal cells—On the inner surface of the fibrous 
layer, interposed between it and the vitreous humour, is a layer 
of perfectly clear, transparent cells, having very thin walls, free 
from nuclei, but which after a time become very delicately gra- 
nular. Hassell describes them as situated on the outer surface 
of the fibrous layer, and thus in the texture of the retina, 
which is certainly wrong, while Bowman considers them as 
part of the hyaloid membrane. Whether they should be 
regarded as part of this or of the retina is doubtful, though, 
as they appear to be more intimately connected with it than 
with the hyaloid membrane, I incline to think them a part 
of it. They appear to be the medium of connexion between 
the two, and serve to organically unite them. When detached 
they are perfectly globular, but when én situ, and particularly 
when somewhat Ealan ged by imbibition, as they become if 
the eye be immersed in fluid for awhile, or by the action of 
its own fluids, they are irregular in outline and overlap each. 
other from disten sion. T hey appear to form a single layer 
of cells. Their size varies much in different animals, the 
size of the creature being no guide to the size of the cells. 
They, however, appear to bear some relation to the size of the 
rods and blood-discs, as these do to each other. They are larger 
in birds than in mammalia, the largest I ever saw being in 
the turkey and in a canary bird, (Pls. X and XI), ; aa these 
are probably enlarged by endosmose. 
