Dr Knox on the Comparative Anatomy of the Eye. 341 
anterior layer of the iris is connected with the inner membrane 
of the cornea, by numerous short fibres, the exact nature of 
which is as yet unknown. From this distribution of the inner 
membrane of the cornea (generally described as the tunic of the 
aqueous humour), it follows, that, during the strong contrac- 
tion of the pupil, in viewing objects placed close to the eye, the 
form of the aqueous humour must be considerably affected. 
In the very lengthened section on the choroides, and its ap- 
pendages, the whole of its structure has been carefully re-exa- 
mined, and, it is hoped, its true anatomy pointed out. Dr Knox 
has endeavoured to prove, that the marsupium in birds and 
fishes is simply a reflected membrane of the choroid, and has 
traced its functions, and the cause of its disappearance, in the 
mammalia. The connection of the ciliary fibres, those numerous, 
delicate, fibrous-looking bodies, placed immediately over the 
capsule of Petit, is shewn to be different from what is usually 
understood ; and that they ought to be considered rather as 
membranous folds, quite analogous to the folds of the inner 
membrane of the choroid coat, and ultimately terminating in 
processes also analogous to those termed Ciliary. The manner 
in which these internal, or colourless ciliary processes, are con- 
nected with the external or true ones, and their importance in 
the anatomy of the humors of the eye, have been demonstrated 
at considerable length. It is obvious, that the details could not, 
with propriety, be introduced into this brief notice.. These mem- 
branous folds have lately been supposed to be muscular, or at 
least it has been conjectured that muscular fibres exist in this 
situation ; but analogy, as well as ocular demonstration, are 
against this opinion. 
As fishes have no true iris, neither have they any ciliary 
nerves. The connection of the iris with the ciliary muscle, and 
with the neighbouring parts generally, is shewn to be somewhat 
different in the eyes of the mammalia from what is usually de- 
monstrated passing around the globe of the eye, which seems 
hitherto to have escaped notice. In birds, this canal is situated 
immediately over the great plexus of nerves, and may readily be 
distended with the blowpipe. In the mammalia it is less in dia- 
meter, but still sufficiently distinct ; it is in some animals partly 
filled up with a very loose dark-coloured cellular membrane. 
