~ 

BIRD’S-EYE VIEWS. « (Ot 
this reason known as the “vitreous humor.” There is much 
less aqueous than vitreous, because the anterior chamber is 
much the smaller of the two; but birds have more of the 
former, compared with the quantity of the latter, than 
mammals, because the size and convexity of the cornea is 
relatively greater. The aqueous humor is enclosed in a 
very delicate simple membrane, that cannot be demonstra- 
ted without difficulty. The vitreous is contained in a more 
palpable, as well as complex membrane—the hyaloid (e) 
—which, besides lining the interior of all the back part of 
the eye, and enclosing the lens as already described, sends 
thin lamin, or layers, all through the vitreous humor, form- 
ing partitions that serve to steady the glassy waters. 
e may next turn our attention to the optic nerve (@) 
that presents itself in the all-important character of the “soul 
of the eye.” It has many peculiarities in birds ; among them 
one that constitutes the most characteristic feature of the eye 
of these creatures. In mammals, as a general rule, the nerve 
is a smooth cylinder that comes straight to the sclerotic, near 
the middle behind, penetrates straight through the coats of the 
eyeball, and then spreads out on all sides to form a disk on 
the inside of the back of the eye. This circular saucer-like 
expansion is the retina—the sensitive nervous plate, or mir- 
ror, upon. which images of things viewed are photographed, 
to be transmitted along the nerve to the brain, and there 
“perceived.” Suppose the optic nerve to be an umbrella- 
handle, the retina would then be the umbrella, blown inside 
out by the wind. In birds the nerve acts very differently. 
In the first place, though it is cylindrical, it is not smooth; 
It has lengthwise folds and ridges. It is like a fluted column. 
It comes obliquely towards the eye, which it strikes at a 
Point eccentric from the axis of the ball; and then, instead 
of at once piercing all the sclerotic coats, and expanding 
‘0 a concavo-convex disk, it tapers gradually to a fine 
Point. This elongated extremity runs still obliquely, down- 
wards and forwards, in a deep groove in the sclerotic, that 
AMER. NATURALIST, VOL. I. 73 

