Page:The Fauna of British India, including Ceylon and Burma (Birds Vol 1).djvu/44

12 the classification of the Passeres—as witness the Tracheophonæ. Briefly three types of syrinx are recognized—the Tracheo-bronchial, the Tracheal and the Bronchial, the last two being derivatives of the first. The tracheo-bronchial is the type found in the Oscines and Sub-Oscines. Herein the lower end of the trachea has the last four or five rings welded to form a little dice-shaped box communicating below with the bronchi. The bronchial rings I and II are closely attached to this box, while III forms a strong arcuate bar supporting a delicate sheet of membrane stretched between rings I and II on the one hand and IV on the other. The bronchial rings are incomplete on their inner aspects, their free ends supporting a "tympanic membrane," which plays an important part in voice

production. At the junction of the bronchi with the trachea is a bony bar—the "pessulus." This supports a thin fold of membrane whose free edge cuts across the bottom of the dice-shaped box of the tracheal tube. By its vibrations it acts like the "free reed" of an organ-pipe. Muscular lips extending from the inner surfaces of bronchial semi-ring III narrow the aperture on either side of the "reed" during the production of the "voice" or song, and thus complete the mechanism of voice production.

In the Tracheal syrinx a variable number of the lower tracheal