82 
Mr. Home's Lecture 
but in all of them the transverse diameter is the longest. In 
the haddock, the proportion is -f^ths to -^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/f in others entirely so,J but in general the pos- 
terior part is membranous, although 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 ralc.|| 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, •f-f 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 acci- 
dents. 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 at- 
tended with advantage, such a -covering is employed as an ex- 
ternal defence. 
In the eyes of fishes, the ciliary processes are entirely want- 
ing. The crystalline lens is spherical, and imbedded in the 
vitreous humour, which is inclosed in cells of a stronger tex- 
ture than in other animals. 
The iris does not admit of motion ; this is taken notice of by 
• Haddock. f Sword-fish. % Devil fish. § Mackerel, 
fl Sword-fish. ** Pike. -j-f Haddock. 
tr 
