THE ANATOMY OF BIRDS.— NEUIIOLOOY. 179 



Birds are therefore far-sighted or near-siglitod (presbyopic or inyopic^l according to the degree 

 of /(■((sioM the nerve-tide excites iu the eye by the mechanism described further on ; and tlie 

 transition from one to the other state is eifected witli great (piickncss and correctness. Ob- 

 serve an eagle soaring ah)ft until lie seems to us but a sjieck in the Idue expanse, lie is far- 

 siglited ; and scanning the earth behiw, descries an object much sniaUer tliau himself, which 

 would be invisible to us at that distance. He prepares to ponnci? upon liis quarry; in the mo- 

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

 aim, and sees well hoAV to complete the bloody work begun. A huuiming-l)ird darts so quickly 

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

 twig. How fiir off it was wlien 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 

 themselves are telegraph-wires and light-houses ; tlmusands 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 mcsetbmoid, but may bo also 

 in part orhito-sphenoidal and pre-spheuoidal. The brim is completed in few birds, by uni(.in of 

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

 appro.^imation 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 pterygxiid 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, .:), 

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

 The 1st — 6th nerves (p. 176) inclusive usually enter the orbit: of their foramina, the optic 

 (figs. 66, 68, 70, 71,2, amj gg, g3^ ,f^ ig mochthe largest and most constant, geuerally blended 

 with its fellow. Those ftir 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 tits and nearly tills it. A bird's eye-hall is much larger than the opening of the 

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



As to its development: "the Ei/e" says Huxley "is fi.irmed by the coalescence of two sets 

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

 brain. The 0])eniug 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 

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

 mis of its floor thickens, and is metamorphosed into the cri/stalline lens : the cavity fills with 

 the aqueous humor. A vascular and muscular iugrowth taking plac(> round the circumference 

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

 around the c<irnea., 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 the cavity of the nose. It may be raised, on its inner side, into a broad fold, the nictitatinq 

 inenthrane, moved by a proper nmscle or muscles. Special glands — the lacrymal externallv, 

 and the harderian on the inner side of the eye-ball — may be dev(doped in connecti(Ui witli, and 

 pour their secretion on to, the conjunctival mucous membra ue. The posteriiu- chamber of the 



