SENSE ORGANS. 



215 



cords in each. (3) Rising up from the fore-and-aft car- 

 tilage formed by the union of the two bronchi, observe 

 a tense membrane with scythe-like edge — semilunar 

 membrane (Fig. 137, C). 



Mode of Action. — The several rings of this apparatus 

 are movable on one another. By appropriate muscles 

 the resonating membrane may be made more or less 

 tense — the vocal cords may be stretched and the lips of 

 the rima pressed together so as to vibrate with various 

 degrees of rapidity and give rise to various notes when 

 the air is driven through them. The cavity of the 

 syrinx, the tense membrane of the bronchi, and the 

 whole cavity of the trachea above act as resonators, in- 



B 

 A 



Fig. 137. — Syrinx : A, front view ; B, side view ; C, section through the 

 lower part of the trachea and between the legs of the bronchi, showing 

 the resonating membrane on the inner side of one bronchus : rm, reso- 

 nating membrane ; sm, semilunar membrane. 



creasing the volume of the sound. The semilunar mem- 

 brane is found only in the best singers, and is supposed 

 to produce the trilling so characteristic of some birds. 



