THE DEVELOPMENT OF BIRDS. 223 
all Birds are provided with large and efficient eyes. Their eye- 
balls are generally longer from within outwards than those of 
other vertebrate animals, and their crystalline lens is but little 
convex. ‘The whole eye is shortest and the lens least flat in 
aquatic Birds and longest in the Owls. 
The sclerotic of birds is not only dense but contains bony plates 
which overlap each other and by their contraction protrude the 
aqueous humour—which is very abundant—and so render the 
cornea more convex. An organ called the marsupium or pecten 
is a vascular membrane which projects into the vitreous humour 
along a line extending from near the entrance of the optic nerve 
to the lens. It seems that this organ can be distended and then 
must help to push the lens forwards. These various telescopic 
arrangements facilitate rapid changes from very long to very 
short sight. They are most needful for such active creatures 
as Birds. A Hawk will suddenly descend a quarter of a mile 
and probably can keep a creature it intends to prey on in focus 
all the time of its descent. The Bird’s eye is indeed the most 
perfect of all. 
The nictitating membrane is drawn out over the eye by a 
muscle which arises from the lower part of the inner side of the 
sclerotic, and thence its tendon winds round the optic nerve and 
passes over the eyeball to be inserted into the third eyelid. By 
its contraction it would compress the optic nerve and so impair 
sight, but that it passes through a tendinous sheath of a quad- 
rate muscle which comes from the back of the sclerotic. When, 
then, the winding muscle acts, the quadrate muscle acts at the 
same time, and draws the tendon away from the optic nerve. 
The lower eyelid is more moveable than in Man and Mammals, 
having its own depressor muscle, and contains a small car- 
tilage. 
Two glands secrete fluid to lubricate the eyeball. One of 
these, the Harderian gland, lies at the inner angle of the eye. 
The other, the lachrymal gland, lies, as with us, at its outer 
angle. 
THe DEVELOPMENT OF BIRDS. 
It would be quite beside the purpose of this work to describe 
in detail the very complicated process by which the germ of a 
Bird transforms itself gradually into the structure of the adult. 
Our object, we believe, will be completely attained by a brief 
