THE ANATOMY OF BIRDS.— NEUROLOGY. 179 
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 
woodeock tears through the thickest cover as if it were clear space, avoiding every obstacle. 
The only things to the aceurate perception of which birds’ eyes appear not to have accommodated 
themselves 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. 150; 
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, wand m, and in some the rim is earried out 
by extra supra-orbital and infra-orbital ossification. There isno bony floor, or only such slight 
seaffolding 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, dof, and fig. 63, 2), 
and others in the cerebral wall, aside from the regular foramina which the nerves pass through. 
The Ist—6th nerves (p. 176) 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. 177) 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 Hye” says Huxley ‘‘is formed by the coalescence of two sets 
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 sae is 
the result. The outer wall of this sac becomes the transparent cornea of the eye; the epider- 
mis of its floor thickens,‘ and is metamorphosed into the crystalline lens ; the cavity fills with 
the aqueous humor. A vascular and muscular 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 laerymal duct, 
with the 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, 
aud 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 
