146 PRACTICAL ORNITHOLOGY. 



The Eye. — The aperture left between the eyelids, when the 

 bird is awake, is of a circular form, half an inch in diameter. 

 There are two canthi, or angles, slightly marked, an anterior 

 or inner, and a posterior or outer. Of the eye itself the cornea, 

 or transparent part, projects considerably, and through it we 

 perceive the iris, an annular membrane of a yellow colour, 

 surrounding a circular space, the pupil, through which the 

 black colour of the interior of the eye is seen. The eye is de- 

 fended externally by the Eyelids, of which there are two, an 

 upper and a lower. Over the latter is a thin ridge, projecting 

 horizontally, and named, from its position, the Supraocular 

 Hidge. It is produced by a thin flat bone, appended to the 

 frontal, and at its outer edge is covered by a dense bare skin, 

 like the cere. The Upper Eyelid is formed externally of skin, 

 covered with small soft feathers, internally of a thin layer of a 

 fibrous structure, and a delicate membrane continuous with 

 that of the anterior surface of the eyeball, and therefore named 

 the conjunctiva. It has a bare crenate margin, fringed exter- 

 nally with small bristles, which are the prolonged shafts of fea- 

 thers. This eyelid is very thin, having no cartilage, and so 

 narrow as not to cover more than a fourth of the eye when 

 closed. The Loicer Eyelid is much larger, covers three-fourths 

 of the eye, and is similarly constructed, but on turning it out. 

 Fig. 2, so as to examine its inner surface, wo find interposed 

 between the fibrous layer and the conjunctiva, a concave, 

 yellowish-white, dense, thin, flexible, cartilaginous plate, of a 

 circular form, which, when the lid is raised, exactly covers 

 the cornea or transparent part of the eye. Its principal object 

 seems to be to give firmness to the eyelid ; but may it not also 

 be intended to prevent, when the bird is asleep, the transmis- 

 sion of light through it I The eyelids are closed by means of a 

 thin orbicular muscle, which surrounds the eye, and is attached 

 to the inner edge of the orbit. A small muscle, the levator 

 palpebral superioris, arising from the upper part of the orbit 

 internally, and attached to the hind part of the upper eyelid, 

 raises it up. The lower eyelid is depressed or drawn down by 

 a corresponding muscle, the depressor palpebrw in/erioris. 



