182 
GENEMAL OBNITHOLOGY. 
but so introduced as to show them up intelligibly. A bird's eye-ball is not nearly so spherical 
or globular as a nianinial's. The globe of the human eye is about a tive-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 hones, the sclerotals, 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 b 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 
thin transparent membrane complet- 
ing the eye-ball in front, like the 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 effect, and its collapse in death. 
The sclerotic coat is lined with the choroid membrane, d, loosely woven of cellular tissue, 
replete with blood-vessels, and painted pitch-black with a heavy deposit of pigment-cells. It 
lines the whole globe as far 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, toward the interior, and 
in so reflecting becomes plaited, as a bag is puckered by pulling the strings. These pleats 
converge upim 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 an tero-posterior section of eye-ball : a, optic 
nerve ; 6, sclerotic, its outer coat ; c, sclerotic, its middle and inner 
coats ; fZ, clioroid ; e, hyaloid ; /, marsupium ; g, cornea ; h, h, bony 
plates between sclerotic layers ; i, i, corrugations of choroid, form- 
ing ciliary processes; fc, k, canal of Petit; /, iris; vi, anterior 
chamber of eye; Ji, 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. 
