THE ANATOMY OF BIRDS.— NEUBOLOGY. 183 



Around the oircumferenco 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 structuix's 

 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 sijsi)ended vertically in the 

 aqueous humor, just in front of the lens. Viewed in front, from the out.«ide, 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 coniea, 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 pupil 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 protnides 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 iridian 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 pupU 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 cormorants are of the latter color. The 

 iris is sometimes pure white, as it is in our common "white-eyed" greenlet, Vireo noveboracensis. 

 In the Californiau woodpecker, Melanerpes formicicorus, the eyes are indifferently (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 fonn the investment, a 

 small space is left between them ; this circular tube around the lens is the canal of Petit, I; 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 dififerent 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 known as the vitreous humor. There is 

 much less aqueous than vitreous; but birds have comparatively more of the former than usual, 



