SIGHT IN FISHES. 475 



in much greater perfection in birds than in any other animals. Vultures, e. (/., 

 fly at great altitudes, and yet discern their peculiar food ; and other birds of 

 prey, which, in their rapid flight, diminish so suddedly the distance between 

 them arid the objects of their pursuit, afford a further illustration of this 

 wonderful power of adjustment. The crystalline lens is flattish, especially in 

 vultures, which are so long-sighted ; but it is rounder in owls, which are ex- 

 tremely near-sighted ; it also becomes progressively more spherical in aquatic 

 birds, according to their subaqueous habits, being less so in cormorants, more 

 so in ducks, and still more so in divers. 



Amongst Reptiles, serpents are destitute of eyelids, the skin being con- 

 tinued over the cornea. In crocodiles, tortoises, and turtles, the two eyelids 

 are well developed, but there are no eyelashes. In the chameleon, the skin 

 forming the eyelids is united into a circular zone with a central aperture. 

 The membrana nictitans and the Harderian gland, when present, are both of 

 large size. The lachrymal apparatus exists in all cases ; the lachrymal gland 

 is very large in the turtle. In the Chelonian reptiles, the sclerotic contains a 

 cartilaginous or bony ring. The pupil in the crocodile and many Ophidia, is 

 elongated vertically ; it is, however, frequently round, and sometimes of a 

 rhomboidal figure. There is sometimes a rudimentary pecten. In the aquatic 

 species, the cornea is flattened, and the lens of a globular shape ; but in the 

 terrestrial species, the lens is flatter ; in Emys, it appears elliptical. When 

 the lens is not globular, the cornea is more prominent. 



The Amphibia are provided with eyelids, and the membrana nictitans is 

 moved by its own muscles. The lachrymal apparatus is absent. In the frog, 

 there are only three recti muscles ; the oblique are absent ; but there is a 

 transverse muscle, which passes directly under the eyeball, and is attached to 

 each side of the orbit. The sclerotic is cartilaginous, the choroid well devel- 

 oped, the iris is said to be motionless, the cornea flat, and the lens sphe- 

 roidal. 



In Fishes generally, the integument passes over the eyes, forming a trans- 

 parent covering ; there are neither eyelids nor lachrymal apparatus, the 

 medium in which these animals live maintaining the cornea moist and clean ; 

 but in the sunfish, and in a few species of sharks, there are eyelids, and, it is 

 said, in the latter, even a membrana nictitans. The eyes are usually of large 

 size, especially in those fish which live at the bottom of the ocean ; they are 

 smallest in those which burrow in the mud. In the Amphioxus, there are 

 two dark spots, on which, according to Vogt, a hemispherical lens is found 

 immediately beneath the integument. The eyeballs of fishes are lodged in 

 very large, cartilaginous, or bony sockets, and are usually provided with four 

 recti arid two oblique muscles ; they are symmetrically placed on the sides of 

 the head, their usual direction being outwards, but sometimes upwards ; in 

 the flat-fishes, owing to the peculiar conformation of the cranium, both eyes 

 are situated on the same aspect of the head. In fishes generally, the eyes are 

 so completely lateral, that the visual field of each must be almost or entirely 

 independent ; and it is remarkable that in these animals, there is no optic 

 commissure, the optic nerves being entirely unconnected, and completely de- 

 cussating, one optic nerve passing over into the opposite optic tract. In the 

 Amphibia, Reptiles, and Birds, the optic nerves, as in the Mammalia, are 

 more or less blended at the optic commissure. 



It may here be mentioned, that the decussation of the optic nerves in the 

 Vertebrata, an arrangement which, not having been met with, even in the 

 highest Mollusca or Annulosa, may be typical of the Vertebrate organiza- 

 tion, seems to be related to the decussation of the motor tracts in the medulla 

 oblongata, which is likewise quite peculiar to the Vertebrate type of struc- 

 ture, not occurring in the Annulose or Molluscous animals. The constant 

 use and importance of sight, in the government of the movements of animals, 

 probably necessitates a co-ordination between the visual and motor organs on 

 the two sides of the body ; but why this cross-action should prevail in Verte- 

 brata, and not in the non- Vertebrata, has not been hitherto explained. A 

 suggestion may here be offered, viz., that it depends upon the inversion of the 

 retinal images within the eyes of the Vertebrata, necessitated by the optical 

 structure of their eyeballs, and by the concave recipient surface of the retina. 



