12 bird's-eye views. 



ficial examination the sclerotic presents itself as a single 

 homogeneous tissue. In the figure (5) 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 maroins advancina: and recedins: with the varvinsr 

 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 fonns 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 

 latter. 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 l)irds of prey (Owls, Sfrigidce). The alteration of 

 shape that the cornea is capable of is next most singular, as 

 Avill 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 is 

 tough and fibrous, with comparatively few blood-vessels, the 

 latter is more loosely woven of cellular tissue, replete with 



