THE ANATOMY OF BIRDS. —NEUROLOGY. 
183 
Around the circumference of the iris, where sclerotic, corneal, and choroid coats come together, is 
a circular band of fibres, the ciliary ligament ; and on the outer surface of the choroid is a similar 
band of circular and radiating contractile fibres, the ciliary muscle. These ciliary structures are 
supposed to be the agents of the accommodating faculty of the eye, acting upon the lens to alter 
its shape or its position, or both. It is a difficult matter to settle, when such delicate structures 
are in question. 
The iris, I, I, or rainbow of the eye, is an exquisite structure hanging like a many-colored 
curtain vertically between the two compartments of the eye ; a highly ornamental framework 
of the eye's window, being both sash and blind to the pupil. It is suspended vertically in the 
aqueous humor, just in front of the lens. Viewed in front, from the outside, the iris appears as 
a colored circular band around the pupil, and seems to come to the surface of the eye. But 
this is not so, for the conjunctiva, the coraea, and the aqueous humor of the front chamber of 
the eye, are between us and it. It may be likened to the dial-plate of a watch, which we look 
-at without noticing the interposed crystal. Similarly, the p'^ipil of the eye, which shows us our 
own reflection, diminished to the size of the ''eye-baby," may be likened to the round central 
hole in the dial-plate through which protrudes the shaft that bears the hands of a watch. The 
'' pupil " is the round black spot within the colored rim of the iris ; but it is not a thing — it is 
a hole in a thing — the hole in the iris through which we may look and see the black choroid 
coat behind. The quivering iris is very similar in texture to the choroid, being a delicate tissue 
of interlacing fibres and vessels ; but it is highly mobilized by circular and radiating sets of 
contractile fibres, by which the curtain is tightened and loosened, with corresponding change 
in the size of the central orifice — the pupil. Although the iridiau movements are largely 
automatic, depending upon the stimulus of light, they are to some extent voluntary, as any one 
may satisfy himself who observes owls in confinement. During these expansions and con- 
tractions of the iris, the pupil in birds preserves its circularity ; and even when the movement 
is freest and most voluntary, as in owls, the contracted pupil never appears as a vertical oval 
figure, or a slit, like that of cats. The round pupil of the great horned owl ranges from the 
diameter of a finger ring down to that of a small split-pea. The iridian colors are often 
striking in birds. Though black and brown are the commonest, yellow is quite frequent, 
red is often seen, blue and green are rarer ; the eyes of connorants are of the latter color. The 
iris is sometimes pure white, as it is in our common "white-eyed" greenlet, Vireo noveboracensis. 
In the Californian woodpecker, Melanerpes formicivorus, the eyes are indifi'erently (or at differ- 
ent ages of the bird, or seasons) brown, bluish, pink, rosy, or yellow. 
The crystalline lens, o, is a transparent biconvex disc, like a common magnifying glass, 
apparently set in the iris like a mirror in its frame, but really hanging a little back of that 
structure. It is enclosed in a capsular membrane, n, of extreme delicacy and transparency, 
which is in turn set between two layers of the hyaloid membrane to be presently noticed. 
Where these layers of hyaloid separate around the rim of the capsule to form the investment, a 
small space is left between them ; this circular tube around the lens is the caiial of Petit, k, k. 
The lens is stationed in the axis of vision ; some suppose it to be equally stationary in any 
transverse axis. It is, however, difficult to understand how an object thus suspended in 
fluctuating humors should be insusceptible of some motion backward or forward, as well as 
of alteration in its degree of convexity ; both of which may be factors in the focusing process. 
From what has preceded, it is evident that the cavity of the eye is divided into anterior and 
posterior compartments, or chambers, by the reflection, from the sclerotic wall, of the choroid, 
hyaloid and iridian structures, which with the lens form a vertical partition. Each chamber 
is filled with a fluid of diflerent density and consistence. That in the anterior or corneal 
chamber is thin and watery, and therefore called the aqueous humor; that in the sclerotic 
cavity is more dense and glassy, and for this reason knoMm as the vitreous humor. There is 
much less aqueous than vitreous ; but birds have comparatively more of the former than usual, 
