on Muscular Motion. 
23 
Haller ; and the reason probably is, that the light in water 
is never too strong for the eye to bear. 
There is a muscle situated between the retina and the scle- 
rotic coat, which is, I believe, common to all fish. This muscle 
is particularly described by Haller ; and its use is stated to 
be that of bringing the retina nearer the crystalline lens, for 
the purpose of seeing objects at a greater distance. Mr. 
Hunter called it the choroide muscle, and has preserved se- 
veral preparations of it. 
This muscle has a tendinous centre round the optic nerve, 
at which part it is attached to the sclerotic coat ; the muscu- 
lar fibres are short, and go off from the central tendon in all 
directions; the shape of the muscle is nearly that of a horse- 
shoe ; anteriorly it is attached to the choroide coat, and by 
means of that to the sclerotic. Its action tends evidently to 
bring the retina forwards ; and in general, the optic nerve in 
fishes makes a bend where if enters the eye, to admit of this 
motion without the nerve being stretched. 
In those fishes that have the sclerotic coat completely co- 
vered with bone, the whole adjustment to great distances 
must be produced by the action of the choroide muscle ; but 
in the others, which are by far the greater number, this effect 
will be much assisted by the action of the straight muscles 
pulling the eyeball against the socket, and compressing the 
posterior part ; which, as it is the only membranous part in 
many fishes, would appear to be formed so for that purpose. 
In fishes, the eye in its natural easy state appears to be ad- 
justed to near objects, requiring some change to adapt it to 
see distant ones ; in this respect differing entirely from the 
bird, the quadruped, and the human. 
