488- 



FISH. 



more dense in fishes than in land animals. Monro 

 found the crystalline lens of an ox to be 1 104, while 

 that of a cod was 1 165, water being reckoned at 1000. 

 The crystalline lens projects through the pupil, and 

 leaves scarce any space for the aqueous humour. The 

 vitreous humour 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 \, crystal- 

 line lens %, and the vitreous humour ^. The spherical 

 form of the crystalline lens has been already alluded 

 to ; but the following table, from the observations of 

 Petit and Cuvier. will exhibit more clearly the pro- 

 portion between the axis and the diameter in a few 

 species. 



The axis to the diameter in the 

 Salmon as . 9 to 10 Whiting . 14 to 15 

 Sword-fish . 25 26 Shark . . 21 22 

 Shad ... 10 11 Ray ... 21 22 

 Pike ... 14 15 Herring . . 10 11 

 Barbel ... 11 12 Tench . . 7 8 



Carp ... 14 15 Eel ... 11 12 

 Mackerel . . 12 13 Conger . . 9 10 



In the eyes of fishes the sclerotic coat is firmer 

 than it is either in mammalia or in birds. It is here 

 cartilaginous, semi-transparent, and elastic, and suffi- 

 ciently 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 cornea, where it sometimes appears like an 

 osseous ring. The outer layer of the choroid coat is 

 either white, silvery, or gold-coloured, and is very 

 thin and a little vascular. The inner coat, to which 

 the term membrana ruyschiana has been applied, is in 

 general black, and covered every where 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 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 con- 

 sidered by some as muscular, and enabling the eye to 

 accommodate the 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 Chondropterygii, as 

 for instance in the rays and sharks. In the eyes of 

 fishes the iris has generally a rich metallic lustre, 

 usually of a golden or silvery tint. This is occasioned 

 by its transparency, which allows the natural colour 

 of the choroid or painted coat to be seen through it. 

 In different species. the pupil or opening of the iris 

 differs in shape ; but, generally speaking, it is either a 

 circle or an oval. In the salmon, and in some other 

 genera, however, it is drawn out into a sharp angle at 

 the fore part ; but what may be the use or effect upon 

 fishes of those differences in the form of the pupil, we 

 have no means of ascertaining, inasmuch as we cannot 

 refer them to certain habits of the animals, as we can 

 in the case of those mammalia which have the pupil 

 contracting in an oval form upon an axis either vertical 

 or horizontal. There is one species of fish, Anableps 

 tetrophthalmus, which has a very peculiar formation of 

 the eyes. This fish has in reality only two eyes, but 

 there is a double pupil to each, which gives it the 

 appearance of having four. See the article ANABLEPS. 



In the skate the upper edge of the pupil is formed 

 into several stripes arranged like radii of a circle, of 

 a golden colour on their external surface, but black 

 on the internal one. In ordinary states of the eye 

 they are folded back and concealed between the 

 upper margin of the pupil and the vitreous humour ; 

 but if pressure is applied to the upper part of the eye, 

 they unfold and come partially over the pupil like 

 the blind of a window. In the torpedo or cramp 

 fish there is a veil of this kind which is capable of 

 entirely covering the eye. Of the bony fishes, and 

 indeed of most of the cartilaginous ones, none possess 

 an apparatus of this kind, though in most fishes there 

 is a vertical veil at the corner of the orbit which 

 covers a small portion of the eye. The eyes of 

 fishes are, generally speaking, supported on a mass 

 of elastic jelly-like matter, which acts as a spring, 

 and prevents them from receiving any concussion 

 even in the most violent motions of the fishes. But 

 in the cartilaginous fishes the support of the eye is 

 quite different from this. In them the eye is placed 

 on one end of an elastic peduncle or stalk of car- 

 tilaginous substance, which has its other extremity 

 inserted in the bottom of the orbit, and the muscles, 

 by acting on the eye placed on this flexible peduncle, 

 can gite it extensive motion in every direction, while, 

 when the muscles cease to act, the elasticity of the 

 peduncle brings the eye back to the mean position. 

 Fishes which have eyes of this description are 

 generally very voracious in their dispositions, and 

 this mobility of the eye gives them great command of 

 all the space around them. 



We have occupied so much space in describing 

 the eyes of fishes, that we can spare but little to their 

 other organs of sense ; but, in respect of them less is 

 necessary, first, because they are much less perfect, 

 and the fish appear to depend less upon them for 

 information and guidance ; and, secondly, because 

 we do not know how ears and olfactory apparatus 

 may act under the surface of water. Sound, as we 

 hear it, is heard by peculiar pulsations of the air ; 

 and scent, we have reason to believe, is occasioned 

 by particles of the odorous body which diffuse them- 

 selves through the air, and though we often expe- 

 rience scents emitted by the air to the water, we 

 know not what may be the effect of similar succes- 

 sions under the surface. When an odorous substance 

 is floating in a running stream, we have no reason to 

 suppose that any particles capable of affecting organs 

 of smell can be given out by it against the current ; 

 and if we may reason from analogy, in a case where 

 we can reason from nothing else, we must be led to 

 conclude that such emanations can make their way 

 through the water only to very short distances, so 

 that in an ample body of water it does not appear 

 that there can be much scent 



Then, in the great body of the oceanic waters 

 which is the grand habitat of the fish a field which 

 man has not either to plough or to sow, but from 

 which he may nevertheless reap, and does reap when 

 even moderately industrious, a most abundant har- 

 vest there are other causes capable of destroying all 

 odorous matter. The sea contains a vast quantity of 

 chlorides, or at all events of hydrochlorides, substances 

 most destructive of the effluvia, especially the noxious 

 effluvia, of bodies in the air, and thus we may con- 

 sider the sea as the grand laver of purification for the 

 globe, and this fact is against the existence of much 

 sense of smell in fishes. 



