672 PHILOSOPHICAL TRANSACTIONS. [aNNO I796. 



cells of a stronger texture than in other animals. The iris does not admit of 

 motion; this is taken notice of by 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 sclerotic 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 several preparations of it. This muscle has a tendinous centre 

 round the optic nerve, at which part it is attached to the sclerotic coat; the 

 muscular 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 it enters the eye, to admit of this motion without the 

 nerve being stretched. 



In those fishes that have the sclerotic coat completely covered with bone, the 

 whole adjustment to great distances must be produced by the action of the cho- 

 roide 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 eye-ball 

 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 adjusted 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. As the change which 

 the eye is to undergo is different, so are the parts which produce it. The cornea, 

 in many fishes, is neither circular, prominent, nor elastic, and the ciliary pro- 

 cesses are wanting. The straight muscles pass off in different directions, to 

 prevent the eye from being pressed on laterally ; the coats of the eye at that part 

 are bony, in some fishes, to prevent the same eflect; and the bottom of the 

 orbit, which in other animals is filled with fat and loose cellular membrane, has 

 no such covering, but is a hard concave surface, to give resistance, and assist in 

 flattening the eye. 



From the preceding observations, deduced from the structure of the eye in 

 different animals, it appears that there are 1 modes of adjusting the eye, one for 

 seeing in air, the other for seeing in water; and it is probably the want of this 

 knowledge that has misled former inquirers, by confining their researches to the 

 discovery of some 1 principle common to the eyes of all animals. The crystal- 

 line lens, as the most conspicuous part, engrossed their whole attention, and 

 they did not think any of the others capable of giving material assistance in 

 producing so curious an effect. The ciliary processes, from their connection 



