OBSERVATIONS ON THE STRUCTURE OF THE RETINA. 101 
In general the more marked the systematic characters are 
in the different divisions of a class of vertebrata, the more do 
we observe variations in the microscopic characters of the 
retina. The retina of the sturgeon presents one of the most 
remarkable examples of this. In a recent examination I 
found that the layer of batons in this fish is constituted in 
accordance with a type foreign to the other fishes, a type 
which occurs moreover in the class of birds. There are two 
elements, the cones and the batons. The latter are trun- 
cated externally, whilst the internal part passes into a conical 
point. The fatty drops, which have been mentioned by 
other observers, do not belong to the batons but to the cones, 
which I had formerly suspected, and as may be seen in my 
work above referred to. The cones are composed of an inter- 
nal thicker, and an external thinner part, as in birds. At 
the extremity of the former part is the fatty drop, which, ex- 
cept in its less brilliant colour, exactly resembles those which 
are found in the cones of birds. We do not at present know 
any other fish, of which the retina exhibits this arrangement 
of cones and batons, exactly similar to that of birds. But on 
the one hand it is very remarkable that this type of the 
retina, proper to birds, also occurs in certain reptiles, namely 
the tortoises, which, themselves, in this respect differ widely 
from the other sections of the reptiles. On the other hand, 
I may remark, that, amongst fishes, it is exactly in the orders 
which also possess the most peculiar characters, that we find 
the most distinct variations in the elements of the retina. In 
the sturgeons the layer of cones and batons is constituted in 
accordance with the type of birds; in the Cylostoma, as ap- 
peal's from my previous researches, there are only simple 
cones, without batons; in the Plagiostoma, on the contrary, 
I have only found batons and no cones. In the class of 
reptiles, we also find very important differences between the 
Batrachia, the Sauria, and the tortoises, whilst in the birds 
and mammalia there is a greater uniformity in the general 
type of the elements referred to, and only slighter modifi- 
cations. 
Another remarkable point is the presence of nervous fibres 
with double outlines in the retina in certain animals. It is 
well known that in the eye of the rabbit there is a beautiful 
white radiation especially on the two sides of the entrance of 
the optic nerve, and many observers have remarked that 
fibres are sometimes found elsewhere which contain a kind of 
medulla. But, besides the rabbits, there are many animals 
in which the optic fibres present a medulla with dark out- 
lines, in a very marked degree. 
XXX. 
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