THE ANATOMY OF BIRDS. — NEUROLOGY. 185 



Birds are therefore far-sighted or near-sighted (presbyopic or myopic) according to the degree 

 of tension the nerve-tide excites in the eye by the mechanism described further on ; and the 

 transition from one to the other state is effected with great quickness and correctness. Ob- 

 serve an eagle soaring aloft until he seems to us but a speck in the blue expanse. He is far- 

 sighted ; and scanning the earth below, descries an object much smaller than himself, which 

 would be invisible to us at that distance. He prepares to pounce upon his quarry ; in the mo- 

 ment required for the deadly plunge he becomes near-sighted, seizes his victim with unerring 

 aim, and sees well how to complete the bloody work begun. A humming-bird darts so quickly 

 that our eyes cannot follow him, yet instantaneously settles as light as a feather upon a tiny 

 twig. How far off it was when first perceived we do not know ; but in the intervening fraction 

 of a second the twig has rushed into the focus of distinct vision, from many yards away. A 

 woodcock tears through the thickest cover as if it were clear space, avoiding every obstacle. 

 The only things to the accurate perception of which birds' eyes appear not to have accommodated 

 tliemselves are telegraph-wires and light-houses ; thousands of birds are annually hurled against 

 these objects to their destruction. 



The orbital cavity^ orbit, or socket of the eye, has been almost sufficiently described (p. 156 ; 

 see also any figs, of skull in profile) as that great recess in the side of the skull bounded above 

 by the roofing frontal bone, behind by this and sphenoidal elements, in front, if at all, by lateral 

 ethmoidal elements (pre-frontal), and separated from its fellow more or less completely by the 

 inter-orbital septum, which is chiefly the perpendicular plate of the mesethmoid, but may be also 

 in part orbito-sphenoidal and pre-sphenoidal. The brim is completed in few birds, by union of 

 lacrymal and post-frontal ; in quite a number of birds, however, it is nearly perfected by the 

 approximation of these same bones, as in fig. 63, u and m, and in some the rim is carried out 

 by extra supra-orbital and infra-orbital ossification. There is no bony floor, or only such slight 

 scaffolding as the expansion of the palatine and pterygoid may afford. The zygoma itself, in 

 many dry skulls, seems like the threshold of the orbital chamber. The bony walls may be also 

 defective in some places by great vacuities in the inter-orbital septum (fig. 70, iof, and fig. 63, z), 

 and others in the cerebral wall, aside from the regular foramina which the nerves pass through. 

 The 1st — 6th nerves (p. 182) inclusive usually enter the orbit: of their foramina, the optic 

 (figs. 66, 68, 70, 71,2, and fig. 63, y) is much the largest and most constant, generally blended 

 with its fellow. Those for nerves 1 and 5 (p. 183) are next most obvious and constant ; others 

 are often, and all may be, thrown into one large opening. In such a socket as this the eye-ball 

 rests upon a cushion of muscle, fat, gland, and connective tissue ; and large as is the chamber, 

 the ball fits and nearly fills it. A bird's eye-ball is much larger than the opening of the 

 eye-lids (see p. 30, note). 



As to its development: "the Eye'''' says Huxley "is formed by the coalescence of two sots 

 of structures, one furnished by an involution of the integument, the other by an outgrowth of the 

 brain. The opening of the tegumentary depression, which is primarily [in the very early em- 

 bryo] formed on each side of the head in the ocular region becomes closed, and a shut sac is 

 tlie result. The outer wall of this sac becomes the transparent cornea of the eye; the epider- 

 Hiis of its floor thickens, and is metamorphosed into the crystalline lens; the cavity fills with 

 tlie aqueous humor. A vascular and nmscular ingrowth taking place round the circumference 

 of the sac, and dividing its cavity into two segments, gives rise to the iris. The integument 

 around the cornea, growing out into a fold above and below, results in the formation of the 

 eyelids, and the segregation of the integument which they enclose, as the soft and vascular con- 

 junctiva. The pouch of the conjunctiva very generally communicates, by the lacrymal duct, 

 with tlie cavity of the nose. It may be raised, on its inner side, into a broad fold, the nictitating 

 membrane, moved by a proper muscle or muscles. Special glands — the lacrymal externally, 

 and the harderian on the inner side of the eye-ball — may be developed in connection with, and 

 pour their secretion on to, the conjunctival mucous membrane. The posterior chamber of the 



