670 



ICHTHYOLOGY. 



Structure game side of. the head, as in flounders, while in others 

 they are nearly vertical. In general, however, they 



{Utictions ' , , ' -l r i J rr>i C 



of Pishes. are P' acet > one on eac " SR ' e f the head. The eyes of 

 \ ^- ~^ fishes are larger in proportion to the size of their 

 bodies than in quadrupeds, as we find the eye of the 

 cod-fish equal in size to that of an ox. 



Fishes in general are destitute of eye-lids, and are sel- 

 dom even furnished with projections in place of eye- 

 brows. In the moon-fish, (Tetraodon mola) however, 

 the eye may be entirely covered with an eye-lid, per- 

 forated circularly. In the greater number of fishes, 

 the skin passes directly over the eye without forming 

 any fold, and in some cases it does not adhere very 

 closely to the eye. Thus the common eel may be 

 skinned without producing any hole in the situation of 

 the eye, the skin only exhibits at that place a round 

 transparent spot. In the trunk-fish, (Ostracion,) the 

 conjunctiva, or external covering of the eye, is so si- 

 milar to the rest of the skin, that we observe lines upon 

 it, which form the same compartments as on the body 

 of the fish. Some fishes may be considered as blind, 

 as the Gastrobranchus coecus, in consequence of the uni- 

 form opacity of the skin in passing over the eye. 



The form of the eye in this tribe of animals is nearly 

 that of a hemisphere, the plane part of which is directed 

 forward, and the convex backward. In the Ray, the 

 superior part is also flattened, so that the vertical dia- 

 meter is to the transverse as 1 to 2. This flatness of the 

 anterior part of the eye is compensated by the spheri- 

 cal form of the crystalline lens. This body is more 

 dense in fishes than in land animals. Monro found the 

 crystalline lens of an ox to be 11 04, while that of a cod 

 was Il6o, water being reckoned at 1000. The crys- 

 talline lens projects through the pupil, and leaves scarce 

 any space for the aqueous humour. The vitreous hu- 

 inour is proportionally small. The portion of the axis 

 occupied by each of the three humours of the eye, in 

 the herring, for instance, may be expressed in fractions 

 as follows : aqueous humour 4, crystalline lens \, and 

 the vitreous humour 4. The spherical form of the 

 crystalline lens has been already stated ; but the follow- 

 ing Table, from the observations of Petit and Cuvier, 

 will exhibit more clearly the proportion between the 

 axis and the diameter in a few species. 



The axis is to the diameter in the 



Salmon as 9 to 10 



Sword-fish 25 



Shad 

 Pike 



10 



14- 



Barbel 11 



Carp 14 



Mackrel 12 



Whiting 14 



Shark 21 



Ray 21 



Herring 10 



Tench 7 



Eel 11 



Conger 9 



11 

 15 

 12 

 15 

 13 

 15 



22 

 11 

 8 

 12 

 10 



The sclerotic coat of the eye of fishes is more firm 

 and dense than in the higher animals. It is here car- 

 tilaginous, semitransparent, and elastic, and sufficiently 

 solid to preserve its form of itself. In the salmon it is 

 of the thickness of a line posteriorly, and of an almost 

 bony hardness before. This is frequently the case in 

 other fishes, especially near its junction with the cor- 

 jaea, where it sometimes appears like an osseous ring. 



The outer layer of the choroid coal is either white, sil- 

 very, or gold-coloured, and is very thin and little vas- 

 cular. The inrier coat, to which the term membrana 

 lluyschiana has been applied, is in general black, and 

 covered everywhere by mucous substance. In the 

 ray, however, it is transparent. Between these two 

 membranes of the choroid coat there is a body of a 

 brilliant red colour. Its form is usually that of a thin 

 cylinder, formed like a ring round the optic nerve; the 

 ring, however, is not complete, a segment of a certain 

 length being always wanting. Sometimes, as in the 

 Perca labrax,. it consists of two pieces, one on each side 

 the optic nerve. It is considered by some as mus- 

 cular, and enabling the eye to accommodate its figure 

 to the distance of the objects ; while others regard it as 

 glandular, and destined to secrete some of the humours 

 of the eye. This gland, we may add, does not exist in 

 the Chondroptcrygii, as the rays and sharks. 



The iris is in genera.1 distinguished by its golden and 

 silvery brilliancy. This arises from its transparency, 

 allowing the natural colour of the choroid coat to be 

 discerned. The pupil is different in form in the differ. 

 nt species, but in general it approaches to circular or 

 oval ; in some genera, as the salmon, it projects into an 

 acute angle at the anterior part. In the Goliitis ana* 

 bleps of Linnaeus, the cornea is divided into two por- 

 tions, and there is a double pupil with a single lens. In 

 the ray, the superior edge of its pupil is prolonged into 

 several narrow stripes disposed in radii, gilded exter- 

 nally, and black internally. In their ordinary state 

 they are folded between the superior edge of the pupil 

 and the vitreous humours : but when we press the su- 

 perior part of the eye with the finger, they unfold them- 

 selves, and cover the pupil like a window-blind. In 

 the torpedo, the pupil can be completely closed by 

 means of this veil. No other fishes possess any thing 

 similar to this conformation, although in most osseous 

 fishes, there is at each corner of the orbit a vertical veil 

 which covers a small part of the eye. 



In general, the eyes of fishes are placed in a conical 

 cup, and repose on a mass of gelatinous matter contain- 

 ed in a loose cellular substance. This trembling elastic 

 mass affords the eye a point of support in all its mo- 

 tions. In the Chondropterygii, however, as the rays 

 and sharks, the eye is joined to the extremity of a 

 cartilaginous stalk, which is itself articulated in the 

 bottom of the orbit. In this manner the muscles act on 

 a long lever, and have therefore great power in moving 

 the eye. 



The optic nerves arise under the cerebrum, and are 

 very large. They are composed either of distinct fila- 

 ments, or of a single flat band, which is sometimes fold- 

 ed longitudinally on itself, and contracted into the fi- 

 gure of a cord. They cross each other without being 

 confounded, and we plainly see that the nerve of the 

 left side proceeds to the right eye, and that of the right 

 side to the left eye. This crossing is less apparent in 

 the cartilaginous fishes, although in the ray the right 

 nerve passes through an opening in the left. These 

 nerves pass directly through the membranes of the eye 

 by a round hole. Internally they form a tubercle, 

 which is papillated in the ray, sharks, and carps. The 

 .radiating fibres which arise from the edges of these tu- 

 bercles to form the retina, are very obvious. In other 

 genera the retina is formed from the edges of two long 

 white caudse, in the same manner as it arises in birds 

 from the single white line. 



The eye is one of the most important organs which 

 fishes are known to possess. It enables them to per- 



