BY THE PBESIDENT. 
119 
as much clearness when near as when remote. But there are 
other modifications in the eyes of birds. The eye as compared 
with the brain is of unusual size, and it. is also of peculiar shape, 
for the antero-posterior axis is considerably lengthened. The 
cornea is prominent, and the retina is further removed from the 
lens than in the human eye, while the surface of the hinder portion 
of the eyeball is larger than that of the front portion. The peculiar 
form of the eyeball is most marked in the nocturnal birds of prey, 
and least in aquatic birds, in which also the cornea is less convex. 
Under the cornea of all birds is a bony ring, which is highly 
developed in owls. In such birds bony plates are arranged side by 
side in the direction of the axis of the eye, and form a case around 
the optic chamber. These bones can be made to overlap each 
other, and, as the case contracts, the cornea and the lens are not 
only rendered more convex, but the distance between the lens and 
the retina is increased. This arrangement permits of acute vision 
of near objects. All these modifications of the vertebrate eye are 
such that the focus of the eye can be made to vary between long 
and short-sighted vision, and also that the animal can see in a weak 
light. All the structures necessary to effect these objects are found 
in a greater degree in birds, such as owls, which are of nocturnal 
habits. 
All birds, with the exception of the wingless Apteryx, have a 
vascular organ, called the pecten■ * which projects boldly into the 
optic chamber close to the entrance of the nerve. It corresponds to 
the processus falciformis of fish, to which I have already referred, and 
its function is believed by some to be confined to the nourishment 
of the inner portions of the eye. Others hold that, when distended, 
it may by means of the vitreous humour alter the shape of the 
optic chamber and assist in adjusting the vision to external 
objects as suggested to be the case with the processus falciformis. 
Birds have three eyelids, two of which correspond to our own, 
but are unlike ours in this respect, viz. that our upper eyelid is 
much more movable than our lower, whereas the reverse is the 
case with birds. Their eyelids rarely have lashes, and, when these 
occur, they may be regarded as feathers rather than hairs. The 
third eyelid of birds is the nictitating membrane, to which I have 
referred as being possessed by sharks. This is situated in the inner 
corner of the eye, and consists of a thin membrane to some extent 
transparent, which can be drawn across the eye as a curtain. It 
* See ‘ Die Sehorgane der Thiere,’ by Carriere, Munich, 1885, and ‘ Unter- 
suchungen liber den Kamm des Vogel’s, by Mikalkovics, Schultze’s ‘ Archiv,’ 
Band ix. The development of this organ is treated of by Balfour in ‘ Com¬ 
parative Embryology,’ vol. ii, p. 411. 
