ISi 



GENEEAL ORNITHOL G Y. 



but so introduced as to show tliein up iuti'lligibly. A bird's eye-ball is uot uoarly so spherical 

 or globular as a uiauiinal's. The globe of tlie lunnan eye is about a tive-sixths segment of a 

 large sphere (sclerotic) with a oue-sixth scguieut of a smaller sphere protrudiug in front (cor- 

 neal). Tlie anterior j)art of the sclerotic of a bird is so prolonged as to be in some cases almost 

 tubular or cylindric, and the corneal protuberance is very convex : the result may be likeued 

 to au acorn wliicli has a sliort blimt liernel in a heavy shallow cup, or to a thick old- 

 fashioned watch with a vi'ry convex crystal. Tliis characteristic shape is fairly shown in 

 the figure; but some birds' eyes are nmcli more tubular in front, — owls' for example. The 

 eye-ball being hollow and tilled with fluids which press in all directions, it is hard to see at first 

 how such a peculiar shape is maintained. But the sclerotic coat is very dense, almost gristly 

 in some cases ; and it is reinforced by a circlet of bones, the sclerotah, h, h ; see also tig. 6i, 

 where the circlet is shown. These are packed alongside each other all around the circumfer- 

 ence of one part of the sclerotic, like a set of splints. Tlu^ large discoidal segment of a bird's 



eye is mostly composed of the mem- 

 brane called fr<im its liardness the 

 nclerotic, — thick, tough, and strong, 

 of a glistening livid color. Three 

 sclerotic coats or layers nuiy be de- 

 monstrated by careful dissection; in 

 the tigure 6 is the outer, c the com- 

 bined middle and inner ones, — mucli 

 exaggerated as to tlunr distinctness. 

 The bony plates lie between the 

 outer and middle coats antericu- to tlie 

 greatest girth of the eye-ball, extend- 

 ing from the rim of the disc nearly 

 or quite to the edge of the cornea. 

 They are a dozen to twenty in num- 

 ber, of oblong squarish shape, taper- 

 ing toward the cornea, around which 

 they are thus circularly dispiosed ; 

 they are jn-otty closely bound to- 

 gether, but the circlet as a whole 

 enjoys some little motion back and 

 forward with the varying convexity 

 of the cornea, g. This last is the 

 thiu transparent membrane complet- 

 ing the eye-ball in front, like the crystal over the fivcc of a watch. It is very protuberant 

 in birds, — even a hemisjihere, or almost tubular. Its structure is uot peculiar in birds; but 

 it is remarkable in this class of creatures not only for its convexity, but for the wide range of 

 the variability in convexity which increased or <limiuished ju-essure of the contained hunujrs 

 may effect, and its collapse in death. 



The sclerotic C(.iat is lined with the choroid mcnihraiic, rf, loosely woven of cellular tissue, 

 replete with bhiod-vessels, aud painted pitch-black with a luvivy deposit of pigmeut-coUs. It 

 lines the whole globe as far forward as the edge of tlie sclerotal bones, where it sjilits in two 

 layers. The inyier choroid layer turns away from the wall of the eye, toward the interitn-, and 

 in so retiecting Ix^'omes plaited, as a bag is puck<'red by iiulliug the strings. These pleats 

 converge upiui th(> rim of the delicate capsule enclosing the lens of the eye, n, and there 

 adhere, forming tlie ciliari/ jjrocesses, j, /. The outer layer also starts away from the cir- 

 cumference of the sclerotic wall, as if to pass directly across the cavity, but ends in the iris. 



Fig. 82. — Vertical nntero-pnsterior section of cye-liall : n. optic 

 nerve; 6, sclerotic, its outer coat; c, sclerotic, its midille and inner 

 coats; r/, choroid ; c, hyaloid ;/, marsupium; ,^, cornea ; //,/i,bony 

 plates between sclerotic layers; (', i, corrnfjations of choroid, form- 

 ing ciliary processes; /.; I:, canal of Petit; /, /, iris; m, anterior 

 chamber of eye; n, capsule of the lens; o, lens; p, posterior cliam- 

 ber of eye. Neither the retina, nor tlie peculiar eheathing of the 

 optic nerve, is shown. The nerve, ra.arsupium, and ciliary processes, 

 not falling in this section, can only be arbitrarily shown. 



