188 



GENERAL ORNITHOLOGY. 



but so introduced as to show them up intelligibly. A bird's eye-ball is not nearly so spherical 

 or globular as a mainuuil's. The globe of the human eye is about a live-sixths segment of a 

 large sphere (sclerotic) with a one-sixth segment of a smaller sphere protruding in front (cor- 

 neal). The anterior part 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 likened 

 to an acorn which has a short blunt kernel in a heavy shallow cup, or to a thick old- 

 fashioned watch with a very convex crystal. This characteristic shape is fairly shown in 

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

 eye-ball being hollow and filled 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 sderotals, h, h ; see also fig. 62, 

 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. The large discoidal segment of a bird's 



eye is mostly composed of the mem- 

 brane called from its hardness the 

 sclerotic, — thick, tough, and strong, 

 of a glistening livid color. Three 

 sclerotic coats or layers may be de- 

 monstrated by careful dissection; in 

 the figure h is the outer, c the com- 

 bined middle and inner ones, — much 

 exaggerated as to their distinctness. 

 The bony plates lie between the 

 outer and middle coats anterior to the 

 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 disposed; 

 they are pretty 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 

 tliin transparent membrane complet- 

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

 in birds, — even a hemisphere, or almost tubular. Its structure is not 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 diminished pressure of the contained humors 

 may eff'ect, and its collapse in death. 



The sclerotic coat is lined with the choroid membrane, d, loosely wwen of cellular tissue, 

 replete with blood-vessels, and painted pitch-black with a heavy deposit of pigment-cells. It 

 lines the whole globe as tar forward as the edge of the sclerotal bones, where it splits in two 

 layers. The inner choroid layer turns away from the wall of the eye, tow^ard the interior, and 

 in so reflecting becomes plaited, as a bag is puckered by pulling the strings. These pleats- 

 converge upon the rim of the delicate capsule enclosing the lens of the eye, n, and there- 

 adhere, forming -the ciliary processes, i, i. 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 aiitoro-posteiior se cticni nf eye-ball : a, optic 

 nerve; b, sclerotic, its outer coat; c, sclerotic, its middle and inner 

 coats; f/, choroid; e, hyaloid ;/, marsupium; </, cornea ; h,h, hony 

 plates between sclerotic layers ; i, i, corrugations of choroid, form- 

 ing ciliary processes; k; k; canal of Petit; /, /, iris; m, anterior 

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

 ber of eye. Neither the retina, nor the peculiar sheathing of the 

 optic nerve, is shown. The nerve, marsupium, and ciliary processes, 

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



