VOL. LXXXVI,] PHILOSOPHICAL TRANSACTIONS. 671 



The eyes of fishes have several peculiarities, and in many respects their struc- 

 ture differs from that which is observed in the quadruped and bird. The mus- 

 cles of the eye, that correspond to the straight muscles in the quadruped, are 4 

 in number; they are however differently placed; they do not surround the eye- 

 ball ; but 2 of them are on that side of the orbit next to the. nose of the fish, 

 the other 2 on the opposite side; their attachment to the eye is close to the edge 

 of the cornea; they do not however pass round the eye-ball towards the posterior 

 part, as in other animals, but are connected with the bones of the head at some 

 distance from the eye on each side; so that they cannot at all compress the eye 

 laterally, they can only pull it backwards by the combined effect of their action. 

 The bottom of the orbit on which the eye-ball rests, is solid, and adapted to it, 

 there being no fat interposed between them as in other animals; and where the 

 eye is removed to a great distance from the skull, and that cannot be the case, 

 there is a strong cartilage projecting from the skull to the bottom of the eye, 

 and that end of it next to the eye is concave, and fitted to the portion of the 

 eye-ball directly opposite the cornea, just above the entrance of the optic nerve. 

 This is considered as a fixed point on which the eye moves; but it will also, 

 from the situation of the muscles, allow the eye to be forced back on it, and the 

 whole eye to be flattened. 



The shape of the eye differs considerably in different fishes, but in all of 

 them the transverse diameter is the longest. In the haddock, the proportion is 

 j-fths to -jSj-ths of an inch, and in some fishes it differs much more. The size 

 of the eye does not correspond with that of the fish; the salmon's eye being 

 smaller than the haddock's. The sclerotic coat is in some fishes membranous;* 

 in some partly bone,-|- in others entirely so,;}: but in general the posterior part 

 is membranous, though the lateral parts are bone.^ The cornea is in general 

 flat, not always circular in its shape, is very thin, made up of laminae, and does 

 not lose its transparency in spirits, appearing like talc.|| In others it is more 

 convex, as in fish of prey; this appears to adapt it to the spherical crystalline 

 lens, which in them lies directly behind it.** The tunica conjunctiva forms the 

 anterior layer of the cornea,-|"|- and in some fishes is quite detached. In the eel 

 there is a transparent horny convex covering, at some distance before the eye, 

 to defend it from external accidents. This covering, to an eye fitted to see in 

 air, would entirely take off the effects arising from change of figure in the 

 cornea; but in water, where no such change could be attended with advantage, 

 such a covering is employed as an external defence. 



In the eyes of fishes, the ciliary processes are entirely wanting. The crystal- 

 line lens is spherical, and imbedded in the vitreous humour, which is inclosed in 



* Haddock. + Sword-fish. {Devil-fish. § Mackerel. || Sword-fish. ** Pike, fl Had- 

 dock, — Orig. 



