EYE. 



205 



of the crystalline, it could move it only 

 obliquely." The experiments and inferences 

 contained in Sir E. Home's paper in the Phi- 

 losophical Transactions for 1796, do not appear 

 to me worthy of any attention. A pecten in 

 an imperfect or rudimentary state appears to 

 exist in fishes and reptiles, and has been noticed 

 by Haller, W. Sommerring, and Dr. Knox. 

 In the article AVES of this work Mr. Oweo 

 has also described the pecten, and to that arti- 

 cle I refer the reader for additional information. 



Of the choroid gland or choroid muscle. 

 The eyes of fishes present several remarkable 

 peculiarities, to be accounted for perhaps from 

 their occasional residence in the obscurity of 

 the deep, and at other times near the surface, 

 exposed to the full blaze of sunshine; they 

 must also be frequently exposed to great pres- 

 sure at considerable depths. The sclerotic is 

 not merely a fibrous membrane, but is strength- 

 ened by a cartilaginous cup, and sometimes 

 even by one composed of bone ; the cornea is 

 generally flat or presenting little of lenticular 

 character; the crystalline lens is spherical, and 

 so dense that its "central part is a hard solid; 

 and the choroid presents the remarkable pecu- 

 liarity which I have now to describe. 



On cutting through the cartilaginous sclerotic, 

 a fluid is found generally interposed between 

 this and the choroid: at least it is so in the 

 genus gadus, (cod, haddock, &c.) The external 

 part of the choroid is formed by a most beau- 

 tiful membrane of a brilliant silver aspect, 

 scarcely to be distinguished from that metal 

 when rough and recently cleaned. On tearing 

 this membrane away, the vascular choroid is 

 exposed, and a red horse-shoe-shaped promi- 

 nent mass, encircling the entrance of the optic 

 nerve, appears. This is the choroid gland or 

 choroid muscle. The veins of the choroid, 

 apparently commencing from the iris, ascend 

 in tortuous inosculating branches, of enormous 

 size compared with the dimensions of the part, 

 and appear to terminate by entering this horse- 

 shoe-shaped organ, but this is not their distri- 

 bution, as it is not hollow. The area enclosed 

 by the organ round the optic nerve does not 

 exhibit the same extreme vascularity. On 

 pulling away a delicate film which covers the 

 organ, it appears composed of lamina or plates 

 divisible into fibres, which run transversely 

 from within outwards, confined into a compact 

 body by the delicate film just spoken of, and 

 a concave depression in the structure beneath. 

 The annexed plate, made from an accurate 

 drawing of a careful dissection, represents the 

 general form and vascularity remarkably well. 



Fig. 130. 



Haller, speaking of the choroid iu fishes, 

 says, " this organ is a fleshy pulp, composed of 

 short columns densely consolidated, resembling 

 red gelatine." Cuvier says, " its colour is com- 

 monly a vivid red, its substance is soft and 

 more glandular than muscular ; at least fibres 

 cannot be distinguished on it, although the 

 bloodvessels form more deeply coloured pa- 

 rallel lines on its surface. Its form is com- 

 monly that of a small cylinder bent like a ring 

 round the nerve, which ring is not, however, 

 complete; a segment of greater or less length is 

 always deficient. Sometimes, as in the Perca 

 labrax, it is composed of two pieces, one on 

 each side of the optic nerve. In other cases it 

 is not in a circle but an irregular curve, as in 

 the Salmon, Tetradon mola, and Cod ; but in 

 the carps and most other fishes it approaches to 

 to a circle. Those who suppose that the eye 

 changes its figure according to the distance of 

 objects, think that this muscle is destined to 

 produce this effect by contracting the choroid ; 

 but it appears to me that the numerous vessels 

 passing out of it should rather lead to its being 

 considered a gland destined to secrete some of 

 the humours of the eye. These vessels are white, 

 fine, very tortuous, and appear to traverse the 

 tunica Ruyschiana; they are well seen in the Te- 

 tradon mola and Perca labrax. In the Cod they 

 are very large, anastomose together, and are 

 covered by a white and opaque mucus. This 

 gland does not exist in the cartilaginous fishes, 

 as the Rays and Sharks, in which it approaches 

 more to the character of the eye iu the Mam- 

 malia, as has already been observed in speak- 

 ing of the tapetum and ciliary processes." D. 

 W. Summering says, " Around the insertion of 

 the nerve is seen a peculiar red, thick, soft 

 body of a horse-shoe shape, respecting which 

 it is doubted whether it be muscular, glandular, 

 or merely vascular. It is undoubtedly ex- 

 tremely vascular, and contains many large, 

 branching, inosculating vessels, forming a 

 proper membrane gradually becoming thin, 

 and terminating at the iris. This vascular 

 membrane constitutes the second or middle 

 layer of the choroid." This description applies 

 to the eye of the Cod. Sir E. Home, in a 

 Croonian lecture published in the Philoso- 

 phical Transactions for 1796, says that Mr. 

 Hunter considered the organ in question to 

 be muscular, and proceeds to state that " this 

 muscle has a tendinous centre round the optic 

 nerve, at which part it is attached to the scle- 

 rotic 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 

 choroid 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 dis- 

 tances must be produced by the action of the 

 choroid muscle; but in the others, which are 

 by far the greater number, this effect will be 



