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 our tongue : in the Pelicans it is obsolete.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 lying 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 -enowned 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 nr 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 



