ECONOMY OF THE BODY 



125 



tympanum. It is again of biological interest to notice that 

 wiiile the syrinx is a new structure, and pecuUar to birds, 

 it is in reahty a transformation and compHcation of the base 

 of the trachea and the beginning of the bronchial tubes. 



The main chamber, as we have mentioned, is called 

 the tympanum. It is traversed horizontally by a projecting 

 bony band called the pessulus, and this forms a partition 



MT. 



S.T.M 



Fig. 31. — Syrinx of raven (after Oven), i. Side view laid open; 

 PE., the bony band or pessulus ; M.S., membrana semilunaris, a fold 

 supported on the pessulus ; m.t.i., membrana tympaniformis interna, 

 the internal wall of a bronchus. 2. Syrinx after removal of muscles ; 

 LTR.R., last modified tracheal ring ; br.r., three first modified bronchial 

 rings. 3. Front view of syrinx with muscles (m.), and sternotracheal 

 muscles (st.m.), extending from the trachea to the sternum ; m.t.i., 

 membrana tympaniformis interna. 



between the anterior openings of the two bronchial tubes. 

 It gives off on each side two arched processes, dorsal and 

 ventral, which run down the bronchial tube. Between 

 these two horn-like processes the membranous wall of the 

 bronchial tube is stretched, forming the membrana tym- 

 paniformis interna. In singing birds there is another 

 membrane, the semilunar, attached to the pessulus, and 

 there may be another on each side of the glottis. These 



