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BIRD’S-EYE VIEWS. 573 
ficial examination the sclerotic presents itself as a single 
homogeneous tissue. In the figure (6) is the outer coat, and 
(c)the middle and inner ones combined. : The osseous plates 
just. mentioned lie between the outer and middle sclerotic 
coats, anterior to the greatest circumference of the eyeball, 
and: nearly or quite extend from the rim of the disk to the 
` edge of the central anterior transparent part of, the ball—the 
cornea. . They are fifteen or twenty in number, of an oblong, 
quadrate shape, broader behind, tapering toward the cornea, 
and so disposed as to form a complete bony circlet around 
the latter. Collectively, they enjoy some little motion, their 
anterior margins advancing and receding with the varying 
convexity of the cornea; but, they cannot individually wab- 
ble, being firmly. bound to.each other by the continuation of 
the sclerotic coats between them. 
The cornea (g) is the thin transparent membrane in front 
that the bird looks through. It forms the anterior part of 
the wall of the eye, and is, in one sense, a continuation of 
the sclerotic ; but its texture is very different from that of the 
er. It is the prominent convex part of the eye,—the 
hemisphere of the small globe that has been already men- 
tioned. Its structure offers nothing peculiar, being essen- 
tially the same as in mammals; but its shape is remarkable. 
Always very convex, it is sometimes still more protuberant, 
being elongated into a sort of cylinder, with a hemispherical 
top.. This tubulation is very great, for example, in the noc- 
turnal birds of prey (Owls, Strigide). The alteration of 
shape that the cornea is capable of is next most singular, as 
will be explained when we come to speak of the powers of 
the eye as a whole. _ It is sufficient here to bear in mind the 
unusual shape of the cornea, and its power of increasing and 
diminishing its convexity. 
_ The sclerotic coat is lined inside with a membrane of very 
different tissue—the choroid (d). While the former 1s 
tough and fibrous, with comparatively few blood-vessels, the 
tter is more loosely woven of cellular tissue, replete with 
mai ET 
