180 
GENERAL OBNITHOLOGY. 
eye has a totally distinct origin. Very early that part of the anterior cerebral vesicle which 
eventually becomes the vesicle of the third ventricle, throws 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 from the folding of the wall of the 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., 1871, p. 79.; 
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 efiected by the uprising of the lower 
lid. These are the principal external diflfer- 
ences between the eyes of birds and mammals. 
The movements of the upper lid in most birds 
are much more restricted than those of the 
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 conjunctival (joining) mucous 
membrane, with interposed connective tissue: 
the lower is also stifiened with a smooth plate, 
the tarsal cartilage. The upper is raised by a 
small muscle, called from its office levator pal- 
pehrce 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 orhiculai'is oculi, which nearly 
surrounds the eye, and whose chief 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 feathers 
answer to eye-lashes. When wide open the orifice of the lids is circular, that is, without the 
inner and outer 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, pearly- white fold of the con- 
junctiva. While the other lids move vertically and have a horizontal commissure, the winker 
sweeps horizontally or obliquely 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 the muscles : a, rectus superior ; b, rectus externus ; 
c, rectus inferior ; d, rectus internus ; e, obliquus 
superior; /, (not lettered) obliquus inferior; g, quad- 
ratus; h, pyramidalis, with its tendon, k, passing 
through a pulley in the quadratus (as shown by the 
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. 
