The respiratory organs of birds are highly specialized. The larynx is rudimentary, and sound is produced by the syrinx, a secondary larynx at the bifurcation of the trachea ; this may be tracheal, bronchial or, most often, tracheo-bronchial. The lungs are small and closely connected with the ribs, while from them numerous large air sacs extend among the viscera, muscles and into many of the bones, which, by being filled with hot air, help to maintain the high temperature and lessen the specific gravity of the body. This pneumaticity of the bones is to a certain extent reproduced by the air sinuses of the skull in crocodiles and mammals. Still, the amount of air in the bones does not neces sarily correspond with the power of flight, for the Ratitae (ostriches and emeus) have very pneumatic bones, while in the sea-gulls they are hardly pneumatic at all.
In mammals the thyroid cartilage becomes an important ele ment in the larynx, and in the Echidna the upper and lower parts of it, derived respectively from the fourth and fifth bronchial bars, are separate (R. H. Burne, Journ. Anat. and Phys. xxxviii.
p. xxvii.). The whole larynx is much nearer the head than in Man, and in young animals the epiglottis projects up behind the soft palate. This prevents the milk trickling into the larynx during suckling, and is especially well seen in the Marsupials and Cetacea, though evidences of it are present in the human embryo. In the lower mammals an inter-arytenoid cartilage is very fre quent (see J. Symington, "The Marsupial Larynx," J. Anat. and Phys. xxxiii. 31, also "The Monotreme Larynx," ib. xxxiv. 90).
The lungs show much variation in their lobulation ; among the porcupines forty lobes have been counted in the right lung, while in other mammals no lobulation at all could be made out. The azygous lobe of the right lung is a fairly constant structure and is situated between the post-caval vein and the oesophagus. It is supplied by the terminal branch of the right stem bronchus and, although it is usually absent in Man, the bronchus which should have supplied it is always to be found. (F. G. P.)