180 



GENERAL ORNITHOLOGY. 



eye has a totally distinct origin. Very early that part of the anterior cerebral vesicle which 

 eventually becomes the vesicle of the third ventricle, tlirows out a diverticulum, broad at its 

 outer, narrow at its inner end, which applies itself to the base of the tegumentary sac. The 

 posterior, or outer, wall of the diverticulum then becomes, as it were, thrust in, and forced 

 towards the opposite wall by an ingrowth of the adjacent connective tissue; so that the primi- 

 tive cavity of the diverticulum* which, of course, communicates freely with that of the anterior 

 cerebral vesicle, is obliterated. The broad end of the diverticulum acquiring a spheroidal shape, 

 while its pedicle narrows and elongates, the latter becomes the optic nerve, while the former, 

 surrounding itself with a strong fibrous sclerotic coat, remains as the posterior chamber of the 

 eye. The double envelope, resulting fi-om the folding of the wall of tlie cerebral optic vesicle 

 upon itself, gives rise to the retina and the choroid coat, the plug or ingrowth of connective 

 tissue gelatinizes and passes into the vitreous humor, the cleft by which it entered becoming 

 obliterated." (Anat. Vert., 1S71, p. 79.J 



Birds alone, of all animate beings, may be truly said to "fall asleep" in death. When 

 the " silver cord " of a bird's life is loosed, the "windows of the soul" are gently closed by 



unseen hands, that the mysterious rites of 

 divorce of spirit from matter may not be pro- 

 faned. When man or any mammal expires, 

 the eyes remain wide open and their stony 

 stare is the sign of dissolution. Only birds 

 close their eyes in dying. At the same mo- 

 ment, the eye sinks and seems to collapse, by 

 the ebbing of its waters. The closure is 

 chiefly effected by the upi-isiug of the lower 

 lid. These are the principal external differ- 

 ences between the eyes of birds and mammals. 

 The movements of the upper lid in most birds 

 are much more restricted than those of tlie 

 lower. The few exceptions are chiefly fur- 

 nished by night birds, as owls, whippoorwills, 

 and others of their respective tribes. The lids 

 consist externally of common skin, internally 

 of a layer of covjunctival (joining) mucous 

 membrane, with interposed connective tissue: 

 the lower is also stiffened with a smooth plate, 

 the tarsal cartilage. The upper is raised by a 

 small muscle, called from its office levator pal- 

 pehrtc superioris, arising from the bony orbit. 

 There is no special lowering nor lifting muscle 

 of the under lid ; the lids close together by the action of the orbicularis oeuK, which nearly 

 surrounds the eye, and whose cliief office is to lift the lower lid; the latter has a small dis- 

 tinct depressor muscle. Birds have no true hairs, but in some kinds modified filiform featliers 

 answer to eye-lashes. Wlien wide open tlie orifice of the lids is circular, tliat is, without tlie 

 inner and fiuter corners (canthi) of almond-eyed creatures like man. There is a third inner 

 eyelid, highly developed and of beautiful mechanism : this is the nictitating membrane, or 

 "winker" (nictito, I wink), a delicate, elastic, translucent, jiearly- white fold of the con- 

 junctiva. While the other lids move vertically and have a horizontal commissure, the winker 

 sweeps horizontally or (jbliquely across the ball, from the side next the beak to the oppo- 

 site. If we menace a bird's eye with the finger, it is curious to see the winker rush out of 

 the corner to protect the ball. Owls habitually sit in the daytime with this curtain shading 



Fig. 81. — Right eye-ball, seen from behind, show- 

 ing tlie muscles : a, rectus superior ; &, rectus externns; 

 r. rectus inferior ; d, rectus internus ; e, obliquus 

 superior; /, {not lettered) obliquus inferior; g, quad- 

 ratus; //. iiyramidalis, with its tendon, Ic, passing 

 through a pulley in the quadratus (as shown by tlie 

 dotted line) to keep it off the optic nerve, i, then passing 

 around the edge of the ball to its insertion in the nicti- 

 tating membrane. 



