36 
The iris is the circular curtain which regulates the amount of light 
admitted, and the aperture in it, or pupil is always circular in 
Birds, and not elongated as in cats. Birds have two ordinary 
eyelids, upper and lower, and a third called membrana nictitans, 
or winking membrane, which is drawn over the front of the eye 
from the inner to the outer side. They have no external ear as 
mammals have, neither have they lips or teeth in the ordinary 
sense. The beak or horny covering of the jaws is wonderfully 
modified in different birds, according to the duties required of it. 
In many birds the tongue is either feebly developed, or is encased 
in horn, so that it can hardly be as useful an organ of taste as is 
our tongue’: in the Pelicans itis obsolete. In some birds, however, 
as in the Woodpecker, it is a very powerful seizing organ, as it is 
protruded with great rapidity by means of a special muscle, and is 
well provided with a sticky secretion, which is given off from a 
large gland, which lyimg underneath the muscle referred to, is 
compressed when this muscle contracts; so that in the Wood- 
pecker just as in the mammal called the Great Anteater, the insect 
prey is easily captured. Many birds swallow small stones which 
are found in the gizzard and evidently help to do the work of the 
absent teeth. The lungs of birds must now receive a little attention 
from us, because of their connection with the air sacs. They are 
of comparatively small volume, and their tubes or bronchi run 
nearly parallel to one another, and open by thin-walled tubes into 
the air sacs. Their lungs cannot be expanded and compressed to 
at all the same extent as ours, because the chest wall is compara- 
tively fixed, and they have no well-developed diaphragm The air 
sacs according fo Professor Owen are found in all birds, with the 
exception of the Apteryx. Our knowledge of their existence is ~ 
primarily due to our venowned fellow townsman, the great Dr. 
William Harvey, whose statue we have near Langhorne Gardens, 
while it is to the distinguished anatomist John Hunter, that we 
owe our knowledge of the very curious fact that these air-passages 
and air-sacs communicate also with the cavities of some of the 
bones of the skeleton. Though these sacs are not by any means 
highly vascular, or supplied with blood-vessels to the same rich 
extent as are the lungs, they are nevertheless of enormous 
importance to the bird, thus: they diminish its specific 
gravity. Again, the air which is taken into the lungs, is, 
in high flying birds, of an extremely low temperature, but 
this air is not only brought into contact with that of the 
lungs, but also with that which has been warmed in these deep air- 
sacs. And again, the air is often very dry, as it is for the ostrich 
on the desert plains of Africa, but the air from the air-sacs contains 
a large amount of moisture. There are nine air-sacs. Four lie in 
