SIGHT. 595 



the sclerotic, and supports the iris : within the sclerotic of some there is fat. In 

 the gigantic ichthyosaurus, which formerly inhabited the ocean, but is now ex- 

 tinct, the same is found. The chamelion has very projecting eyes, to which 

 the light is admitted through a minute vertical slit in the skin, which forms but 

 one lid, the upper and lower being united ; and each eye can be turned in a 

 variety of directions independently of the other : there is also a membrana nic- 

 titans nearly as large as in birds. 



The wonderful adaptation which is observed throughout nature, in the most 

 minute point, to the purpose for which every thing was intended, presents us in 

 birds with eyes the most remote in form and structure from those of fishes, and 

 exactly fitted for the rare medium which they inhabit ; the difference between 

 which and the aqueous humour of the eye is so great, that this is sufficient to 

 refract the rays of light powerfully. The aqueous humour is, therefore, so 

 copious that the cornea, which is thin, is very convex ; and, to prevent the 

 sclerotic from being unduly expanded, and the prominence of the cornea lost, a 

 circular series of fifteen or twenty quadrangular, moveable, imbricated bony 

 plates are placed around its edge, between the two plates of the sclerotic : from 

 being imbricated, they are much less liable to fracture than a bony ring. The 

 tough posterior part of the sclerotic forms a large hemisphere almost occupied 

 by the abundant vitreous humour ; while the lens, not being so requisite for re- 

 fraction, is flattened, smaller, and less dense. The iris and lens are necessarily 

 at a great distance from the cornea, by which arrangement the refracting powers 

 are susceptible of great variation, and the animal sees well at the most different 

 distances. Not only were these powers necessary, but a large field of vision ; 

 and therefore the cornea is very prominent, and the eyes of great size compared 

 with the head, and placed laterally upon it. Yet they are less moveable than in 

 quadrupeds ; but the reason of this is the length of their neck and extreme mo- 

 bility of their head. The chorioid coat, lined with a black pigment of globules 

 with a transparent centre, sends a prolongation called marsupium or pecten, 

 puckered and folded, from the entrance of the optic nerve through the vitreous 

 humour, and to the capsule of the lens, for some purpose unknown, unless it 

 be to supply copious vessels for secretion, to darken the eye in the blaze of the 

 sun, or, if muscular, to affect the shape and position of the lens, and thus 

 adapt it for vision in great variations of distance. The iris has various bright 

 colours, and is wonderfully mobile, and thought to be voluntary, in parrots. The 

 membrana nictitans, or moveable semitransparent fold of the conjunctiva at the 

 inner corner of the eye, is of great size and moved by two strong muscles in 

 order to keep the tears constantly diffused over the eye in the rapid course of 

 birds through the air, and to protect it fully against sudden light. It returns 

 to its place by mere elasticity. We have a rudiment of it in our own inner 

 canthus. The eyes of mammalia agree generally with our own ; but, as some 

 of this class fly, some inhabit the water, some are amphibious, and some burrow- 

 in the dark, they are necessarily of great variety. They are generally small in 

 those bulky animals whose food requires not to be discovered at a distance, as the 

 elephant, rhinoceros, and hippopotamus : the eye of the elephant is not above two 

 inches in diameter ; of the whale, not above the 200th part of the length of its body. 

 They are small in moles and shrews ; so small in the mole, that Dr. Magendie 



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