NUNNELEY, ON THE RETINA. 221 
transverse striz and look like connected discs, then granular, 
and ultimately break up into transparent granules, and alto- 
gether disappear, so that in six, twelve, or twenty-four hours 
after death, hardly a straight, clear cylindrical rod is to be 
seen, and frequently in forty-eight hours or less not a trace 
of them is left. 
They are very flexible, and may be seen to bend on en- 
countering any obstacle—as when detached from each other 
they float about—and immediately this is passed, to again 
become straight ; but I do not think they are elastic, that is, 
compressible. They are at the same time, very brittle, and 
most easily break. They appear neither to attract nor repel 
each other, but when brought imto contact they often adhere 
by the parts which actually touch, so that if they happen to 
come end to end, two may easily be mistaken for a single long 
rod. They are largest in reptiles generally, and of these 
in the frog; next in fish, and smallest in man and mamma- 
lia. Though their size varies in different creatures, it bears 
very little proportion to the size of the animal or of the eye- 
ball. Thus the rods of the frog, toad, and water-newt are 
as large or larger than those of the alligator; of the sparrow 
and canary bird as those of the fowl or turkey; of the 
duck as those of the swan; of the carp, trout, eel, and 
herring as those of the halibut, salmon, haddock, and cod. 
Those of the rat, mouse, and mole are nearly as large as 
those of the sheep, ox, and horse; of the monkey as those 
of man; indeed those of man, if anything, are smaller, 
and, I think, more numerous than in almost any other 
creature. 
I have stated that I believe these in the living eye to be 
straight cylinders, but this is not the usual description. 
Hassall has described and figured them as curled into a 
knob at the outer extremity. This is a very common ap- 
pearance when the eye has not been examined until the 
animal has been dead a short time, but certainly does not 
exist in the living eye; and may often enough be seen to 
form while the examination is going on; especially on the 
addition of dilute spirit, or after the unopened eye has been 
immersed in it for a short time. On the other hand, Han- 
nover, in his beautiful plates (‘ Recherches microscopiques sur 
le Systeme nerveux’) has described and figured all the rods 
as terminating outwardly in a conical extremity; this being 
in some animals prolonged into a long delicate filament, 
which is, he says, received into a minute sheath formed in 
the choroid coat, whereby the two tissues are organically 
connected together. In this he is followed to some extent 
by Mr. Bowman. 
