NUNNELEY, ON THE RETINA. 233 
parent cells with large eccentric nuclei and nucleoli, identical 
with those found in the cerebral convolutions. It is very 
difficult to see these brain-cells in situ in the fresh eye, but 
floating about they are readily seen; and in a successful 
examination they may be seen forming a layer in which the 
fibres of the optic nerve are imbedded and expanded. Plate X. 
In this layer, Bowman, Kélliker, and Hassall all describe, 
as being found, besides the circular brain-cells, caudate gan- 
glionic cells. Hassell says only to be found in man; Bow- 
man in man and the horse, amongst mammalia, but parti- 
cularly well developed in the turtle; while Kolliker figures 
those from the ox with very long processes, and his descrip- 
tion would inferentially lead to the supposition of their being 
generally distributed. Bowman says they do not contain any 
pigment, while Hassall represents them as altogether dark, 
and Kolliker as containing pigment in the body of the cells, 
with the large central nucleus transparent. Hannover, in 
his elaborate account of the retina, makes no allusion to any 
such cells, yet he was perfectly familiar with the character of 
caudate nerve-cells, for he has figured them as seen in dif- 
ferent parts of the brain and spinal marrow, and therefore 
we may conclude he either had not found them or does not 
believe in their existence. 
I have searched most carefully, over and over again, for 
these long caudate cells in the eyes of man, many mammalia, 
various birds, reptiles, and fish, and particularly in the almost 
living eye of the turtle, and must confess, ike Hannover, have 
failed to find them in the perfectly recent eyes. When reagents 
are employed, when the retina has been dried and moistened 
with water, or the retina examined is not from an animal just 
dead, not the same difficulty exists; large, irregular, more 
or less caudiform celis are then abundant enough. I am 
therefore, unwilling as I am not to see what such competent 
observers speak unhesitatingly of, constrained to doubt if 
cells such as figured and described, with many long caudate 
processes, continuous with the nerve-fibres, do really exist in 
the living eye. 
There are, however, constantly found, particularly in the 
eyes of mammalia, cells of various sizes ; some large, very much 
resembling in form, only perfectly transparent, epithelial cells. 
They are tiat, irregular, contain nuclei and fine granules, le 
singly or in groups overlapping each other, and are in con- 
nexion with, if not imbedded in, as I believe, the granular 
layer, which they so much resemble, that unless caught de- 
tached from it, at first, until the eye is familiar with their 
indistinct outline, they are very difficult to recognise. I give 
figures of these cells from the retina of the human feetus, the 
