every bird of flight which has this pneumatic condition of the bones; and Dr. Crisp stated that quite a number of birds--the Swallow, Martin, Snipe, Canary, Wood-wren and Willow-wren, Whinchat, Glossy-starling, Spotted-fly-catcher, and Black-headed Bunting--have no air in their bones. And it is well known that in many birds, especially water birds, it is only the upper bones of the limbs which are pneumatic, while the smaller bones retain the marrow. LUNGS AND AIR CELLS [Illustration: FIG. 16. LUNGS OF THE BIRD APTERYX PARTLY OPENED ON THE RIGHT-HAND SIDE The circles are openings of the bronchial tubes on the surface of the lung. The notches on the inner edges of the lungs are impressions of the ribs (After R. Owen)] [Illustration: FIG. 17. THE BODY OF AN OSTRICH LAID OPEN TO SHOW THE AIR CELLS WHICH EXTEND THROUGH ITS LENGTH (After Georges Roché)] It may be well to remember that the lungs of a bird are differently conditioned from those of any other animal. Instead of hanging freely suspended in the cone-shaped chamber of the thorax formed by the ribs and sternum, they are firmly fixed on each side, so that the ribs deeply indent them and hold them in place. The lungs have the usual internal structure, being made up of branching cells. The chief peculiarity consists in the way in which the air passes not only into them, but through them. The air tube of the throat of a bird, unlike that of a man, has the organ of voice, not at the upper end in the form of a larynx, but at the lower end, forming what is termed a syrinx. There is no evidence of this in a fossil state, although in a few birds the rings of the trachæa become ossified, and are preserved. But below the syrinx the trachæa divides into two bronchi, tubes which carry the ringed character into the lungs for some distance, and these give off branches termed bronchial tubes, the finer subdivisions from which, in their clustered minute branching sacs, make up the substance of the lung. There is nothing exceptional in that. But towards the outer or middle part of the ventral or under surface of the lungs, four or five rounded openings are seen on each side. Each of these openings resembles the entrance of the air cell into a bone, since it displays several smaller openings which lead to it. Each opening from the lung leads to an air cell. Those cells may be regarded as the blowing out of the membrane which covers the lungs into a film which holds air like a mass of soap bubbles, until the whole cavity of the body of a bird from neck to tail is occupied by sacculated air cells, commonly ten in number, five on each side, though two frequently blend at the base of the neck in the region of the #V#-shaped bone named the clavicle or furculum, popularly known as the merry-thought. Most people have seen some at least of these semi-transparent bladder-like air cells beneath the skin in the abdominal region of a fowl. The cells have names from their positions,