NUNNELEY, ON THE RETINA. 229 
the end of the rod, and which might give rise to the appear- 
ance; and in the turtle I have seen a few of the globules 
with a similar minute projection on one side, as though it 
had been imbedded in the conical rod; in the swan, where 
the globules are not particularly well developed, I have found 
some few fusiform in shape. 
The use of these globules I can offer no conjecture upon. 
Internally the rods pass in amongst the granules, and the 
end of each rod appears to rest upon and be connected with 
one or more granules. Externally the end of the rod is, I 
think, implanted upon the cellular or epithelial, as it has 
properly been called, layer of the choroid; the clear portion, 
which has been regarded as the choroidal cell, being that 
where the rods have adhered; for there can, I think, be no 
doubt that the retina and choroid are organically connected 
together, the rods probably deriving their nutriment from 
the choroidal vessels ; no blood-vessels are to be found in the 
columnar layer itself. 
The rods are evidently a nervous structure, sui generis, 
their whole appearance indicates this, and probably they are 
intimately connected with the sense of vision; but in what 
way they act, whether as independently perceiving the image, 
which the nervous fibres only place in relation with the 
sensorium, which then intelligibly appreciates the informa- 
tion, or only as affording a suitable surface for the im- 
printing the image upon, which the nerve-fibres then convey 
to the sensorium to be there perceived, it is premature to 
discuss; but their complex character would rather point to 
their possessing some independent function, than to their 
being the mere recipient surface for an image, to be thence 
conveyed to the brain as the sole sentient part. Whether 
they are the real terminal expanse of the optic fibres, as 
Kolliker seems to suppose that Miiller’s and his own 
observations indicate, is, to say the least, unproved; nor 
does it appear to be necessary for our estimate of the im- 
portance of their functions. That they are in connexion 
direct or indirect is more than probable, but that they are to 
be regarded as the expanded nerve-ends does not seem so 
probable as their being peculiar nervous structures, having 
separate and individual power. Where there is the greatest 
amount of intelligence they appear to be the smallest and 
most numerous. 
This is the structure which was first pointed out as a 
distinct layer of the retina by Dr. Jacob, and has since been 
known as the membrane of Jacob; though its nature was 
not, indeed could not have been, known by him. He 
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