THE SYEINX. 



249 



osseous band the pessulus which forms a vertical septum between 

 the anterior apertures of the two bronchi. This septum, at its 

 anterior (ventral) and posterior (dorsal) ends, gives off on each side 

 two arched processes, which pass downwards one along the dorsal, 

 and the other along the ventral edge of the bronchus of its side ; 

 and between these cornua the internal wall of each bronchus, which 

 is here membranous, is stretched, and constitutes the membrana 

 tympaniformis internet,. In the Singing Birds there is in addition a 

 semi-lunar fold (membrana semilunaris) on the pessulus, as a prolonga- 

 tion of the membrana tympaniformis interna. In many cases a mem- 

 branous fold the membrana tympaniformis externa is developed 



a 



FIG. 657. Lower larynx of Raven (from Owen), a, Side view of larynx laid open. b, 

 Larynx after removal of muscles. c, Larynx with muscles, from the front ; d, from the 

 side. St, pessulus ; Mty, membrana tympaniformis interna ; Ms, membrana semilunaris; 

 Rt , modified last tracheal ring ; Rb, the modified three first bronchial rings ; M, muscles. 



on the external side of the tympanum, and forms. with the free edge 

 of the internal tympamform membrane (i.e., with the membrana 

 semilunaris), a vocal slit or glottis on either side. The tension of 

 these folds, which function as vocal cords, is regulated by a muscular 

 apparatus, which connects the trachea with the lateral parts of the 

 tympanum, or also with the anterior bronchial rings, and is most 

 highly developed in the singing birds, in which the syrinx may 

 possess five or six pairs of such muscles. 



The bronchi are relatively short and lead, at their entrance 

 into the lungs, into a number of wide membranous bronchial 

 tubes, which traverse the pulmonary tissue. The lungs are not, as 

 in Mammals, freely suspended in a closed thoracic cavity and invested 



