314 THE ANATOMY OP VEETEBRATED ANIMALS. 



the following characters. The hindermost rings of the 

 trachea coalesce, and form a peculiarly shaped chamber, 

 the tympanum. Immediately beyond this, the bronchi 

 diverge, and from their posterior wall, whei'e one bronchus 

 passes into the other, a vertical fold of the lining membrane 

 rises, in the middle line, towards the tympanum, and forms 

 a vertical septum between the anterior apertures of the two 

 bronchi. The anterior edge of this septum is a free and 

 thin membrana semilunaris, but in its interior a car- 

 tilaginous or osseous frame is developed, and becomes 

 united with the tympanum. The base of the frame is 

 broad, and sends out two comua, one along the ventral, 

 and the other along the dorsal, edge of the inner wall of 

 the bronchus of its side ; which, in this part of its extent, 

 is membranous and elastic, and receives the name of the 

 membrana tympaniformis interna. 



The bronchial " rings " opposite this are necessarily in- 

 complete internally, and have the form of arches embracing 

 the outer moiety of the bronchus. The second and third 

 of these bronchial arcs are freely moveable, and elastic 

 tissue accumulated upon their inner surfaces, gives rise to 

 a fold of the mucous membrane, which forms the outer 

 boundary of a cleft, boiinded, on the inner side, by the mem- 

 brana semilunaris. The air forced through these two clefts 

 from the lungs sets their elastic margins vibrating, and 

 thus gives rise to a musical note, the character of which is 

 chiefly determined by the tension of the elastic margins and 

 the length of the tracheal column of aii-. The muscles, by 

 the contraction of which these two factors of the voice are 

 modified, are extrinsic and intrinsic. The former are pos- 

 sessed by birds in general, and are usually two pair, passing 

 from the trachea to the furcula and to the sternum. Some 

 birds possessing a broncho-tracheal syrinx such as has been 

 described, as the Alectoromorphce, Clienommplue, and By- 

 sporomorphoB, have no intrinsic muscles. Most others have 

 one pair, attached, one on each side, to the rings of the 

 trachea above, and to the tympanum, or the proximal 

 bronchial arcs, below. The majority of the CoracomorphoB 



