222 NUNNELEY, ON THE RETINA, 
For this cone at the outer extremity of the rods, and its 
surmounting filament, I have searched most diligently, 
indeed I may almost say wishfully, as desiring to see what two 
such authorities have described as the true form, but I must 
confess to have failed. True, a short conical appearance is 
often to be seen at one or other extremity of the rod, but I 
have never been able to satisfy myself that it is not an optical 
effect, from the end not being in focus, for almost invariably 
I have found that by focusing the conical appearance disap- 
pears, when the rods are single, and while they are in situ ; 
supposing them to be conical, and the cone enclosed in a 
sheath of the choroid, I do not see how it is to be observed, 
at least I have failed in the fresh eye, and so distorted do 
these smali bodies become by drying the retina, and subse- 
quently moistening it, or by the addition of any reagents, 
that I place very little reliance upon appearance then pre- 
sented; nor do I think the statement of these observers, that 
the great disposition there is for the rod to break at the pre- 
cise point where it becomes conical, is sufficient to account 
for the difficulty of finding the cone; for in this case the 
cones ought to be found separate in some abundance, consi- 
dering the enormous multitude of rods; which they are not. 
Besides which, while they state that the inner end of the rod 
is perfectly straight, and the outer is conical and pointed, im- 
bedded in the choroid, Hassall describes the outer ends of 
the rods as of a globular or oval shape, and Kollker declares 
the opinion of the conical form of the outer end to be a mis- 
take—that, in reality, it is perfectly flat and straight, while 
the inner end is not only conical, but sends a filament so 
prolonged as to pass entirely through the outer layers of the 
retina to the inner surface, where it terminates in radiating 
fibres as first described by H. Miller. Kolliker would ap- 
pear to regard the rods as cells filled with fluid, for he says, 
that on breaking up, “ they allow clear drops to exude, which 
are often met with on the external surface of the retina in 
vast quantity.’ This is contrary to the observations of most 
others, and does not correspond with what the rods appear 
to my eye, as above stated. By some the rods are described 
as cylinders, terminating in an expanded head like a nail; 
very often rods may be seen of this shape. I have seen this 
particularly in the rat ; but it simply arises from the granule 
to which the inner end of the rod is attached still adhering to 
it; by watching for a while, it will be seen to become de- 
tached ; often too they appear conical or swelled out, from a 
granule adhering to some other part of them. 
In all birds, and in the turtle, the straight cylindrical rods 
