THE CLASS BIEDS. 303 



simple structure, and has but little to do with the formation 

 of sounds. Its orifice has the form of a fissure (g, Fig. 247) ; 

 but towards the lower extremity of the trachea is the true 

 larynx, most remarkable, as might be expected, in singing 

 birds. This complex apparatus will be best understood by a 

 reference to Figs. 252 and 253. It may be compared to a 

 kind of osseous drum, the interior of which is divided inferiorly 

 by a traversing beam of the same nature, surmounted by a thin 

 semilunar membrane (c, Fig. 253). This drum communicates 

 inferiorly with two apertures of the glottis (rimee glottidis), 

 formed by the termination of the bronchi, and each provided 

 with two lips or vocal cords; finally, muscles, whose num- 

 bers vary with the species, extend between the different rings 

 of which these parts are composed, and move them so as to 



Fig. 252.* Fig. 253t 



stretch more or less strongly the membranes they support. 

 In birds which do not modulate the sounds in a complex 

 way, the membranous septum is wanting; in birds which 

 do not sing, there are no muscles proper to the inferior larynx, 



* Inferior larynx of the rook : t, trachea ; t', drum formed by the lower 

 end of the trachea ; I, middle ossiculum of the trachea; b', first ring of the 

 bronchi, separated from the third ossiculum of the larynx by a membranous 

 space ; 6, bronchi ; m, proper muscles of the larynx : these muscles have 

 been removed on the opposite side ; m', depressor muscles of the trachea. 



t Vertical section of the larynx : t, inferior portion of the trachea divided 

 as regards the half ; c, semilunar membrane, situated above the point of 

 reunion of the two glottides, and fixed to the osseous cross-beam (o) ; a, little 

 rim formed by the internal lip of the right glottis ; me, inner surface of the 

 right bronchus, formed by a tympaniform membrane; b, portion of the 

 cavity of the right bronchus, exposed by a section of a part of this membrane. 



