SEC. IV ANATOMY OF BIRDS 265 



appear not to have accommodated themselves are telegraph-wires 

 and lighthouses ; thousands of birds are annually hurled against 

 these objects to their destruction. 



The orUM cavity, orbit, or socket of the eye, has been almost 

 sufficiently described (see also any Figs, of skull in profile) as that 

 great recess in the side of the skull which is bounded above by the 

 roofing frontal bone, behind by this and sphenoidal elements, in 

 front, if at all, by lateral ethmoidal elements (prefrontal), 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 orbitosphenoidal and pre- 

 sphenoidal. The brim is completed in few birds, by union of lacry- 

 mal and postfrontal ; in quite a number of birds, however, it is 

 nearly perfected by the approximation of these same bones, as in 

 Fig. 63, « and m, and in some the rim is carried out by extra supra- 

 Orbital and infraorbital 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. 261) 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. 261 ) 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 eyeball 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 eyeball is much 

 larger than the opening of the eyelids (see p. 45, note 3). 



As to its development : " the Eye," says Huxley, " is formed by 

 the coalescence of two sets of structures, one furnished by an involu- 

 tion of the integument, the other by an outgrowth of the brain. 

 The opening of the tegumentary depression, which is primarily [in 

 the very early embryo] 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 epidermis of its floor thickens, and is metamorphosed into the 

 crystalline lens ; the cavity fills with the aqueous humour. 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 segrega- 

 tion of the integument which they enclose, as the soft and vascular 



