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, and on each side one is abdominal, two are thoracic, one clavicular, and one cervical, which last is at the base of the neck. The clavicular and abdominal air cells are perhaps the most interesting. The air cell termed clavicular sends a process outward towards the arm, along with the blood vessels which supply the arm. Thus this air cell, entering the region of the axilla or arm-pit, enters the upper arm bone usually on its under side, close to the articular head of the humerus, and in the same way the air may pass from bone to bone through every bone in the fore limb. The hind limbs similarly receive air from the abdominal air cell, which supplies the femur and other bones of the leg, the sacrum, and the tail. But the joints of the backbone in front of the sacrum receive their air from the cervical air sac. The air cells are not limited to the bones, but ramify through the body, and in some cases extend among the muscles. A bird may be said to breathe not only with its lungs, but with its whole body. And it is even affirmed that respiration has been carried on through a broken arm bone when the throat was closed, and the bird under water.
FIG. 17. THE BODY OF AN OSTRICH LAID OPEN TO SHOW THE AIR CELLS WHICH EXTEND THROUGH ITS LENGTH
(After Georges Roché)
Birds differ greatly in the extent to which the aircell system prolonged from the lungs is developed, some having the air absent from every bone, while others, like the Swift, are reputed to have air in every bone of the body.
Comparison shows that in so far as the bones are the same in Bird and Ornithosaur, the evidence of the air cells entering them extends to resemblance, if not coincidence, in every detail. No living group of animals except birds has pneumatic limb bones, in relation to the lungs; so that it is reasonable to conclude that the identical structures in the bones were due to the same cause in both the living and extinct groups of animals. It is impossible to say that the lungs were identical in Birds and Pterodactyles, but so far as evidence goes, there is no ground for supposing them to have been different.
THE LUNGS OF REPTILES
FIG. 18. THE SIDE OF THE BODY OF A CHAMELEON
Ribs removed to show the sacculate branched form of the lung
