ORGANS OF SENSE. 59 



layers, between the middle and outer of which, from 15 to 17 osse- 

 ous quadrangular plates are placed anteriorly, a repetition of what 

 we have seen in reptiles and fishes; these scales overlap each other, 

 they are connected by the sclerotic, and are capable of a limited 

 degree of motion. The cornea is dense, and possesses a consider- 

 able degree of convexity, as well from the abundant humours as 

 from the pressure of the surrounding muscles ; and, according to 

 the discovery of Crampton, it is capable of varying its convexity 

 by the action of a muscular sphincter attached to the posterior 

 layer at its circumference. The choroid is the same as in mam- 

 malia: the iris is remarkable for the freedom of its motions which, 

 in some instances, seem voluntary: the pupil is generally circular, 

 but elongated transversely in the goose and dove, and vertically 

 oval in the owl. The chief peculiarity in the eye of birds consists 

 in the marsupium nigrum or pecten plicatum ; this wedge-shaped 

 body, which in appearance simulates choroid, is composed princi- 

 pally of vessels, and extends from the entrance of the optic nerve 

 through the vitreous humour towards the lens which it sometimes 

 reaches, and gets an attachment to. Petit supposed that this sub- 

 stance rendered objects in front of the eye more distinct by absorb- 

 ing the lateral rays of light: Home, that by its muscularity it 

 retracted the lens; but this is impossible when it does not reach it. 



Owen considers it an erectile organ, destined to push forward 

 the lens either directly or through the medium of the vitreous hu- 

 mour ; others look upon it as a gland for secreting the vitreous 

 humour, and many are of opinion that it is placed there for the 

 purpose of absorbing the super-abundant rays of light during the 

 exposure of the eye to the blazing sun when soaring aloft. 



The aqueous and vitreous humours present no peculiarities ; 

 the lens is flat in high-flying birds of prey, and more convex in 

 the aquatic species ; it is not here as in the other classes an achro- 

 matic lens, in consequence of the absence of central nucleus. The 

 globe of the eye is moved by four recti and two oblique muscles. 

 The lower lid is the more movable in birds and is provided with a 

 distinct depressor muscle. The third eyelid, or membrana nicti- 

 tarts is a conjunctival fold connected with the inner angle of the 

 eye and capable of being moved across the organ by a peculiar 

 pair of muscles. One of these, from its shape, is named quadratics ; 

 it arises from the upper and back part of the sclerotic; its fibris 

 descend in a convergent manner towards the optic nerve, a little 

 above which they terminate in a free semilunar aponeurotic margin, 

 containing a canal. The second muscle called pyramidalis arises 

 from the inferior internal and posterior part of the sclerotic, its 

 fibres pass converging to the upper surface of the optic nerve, where 

 it forms a slender tendon, which passes though the canal in the 

 quadratus; it then passes from the outer to the lower surface of the 

 sclerotic, conducted in a cellular sheath till it gains the free margin 

 of the membrana nictitans, into which it is inserted. 



By the simultaneous action of this pair of muscles the membrane 



