LARYNX OF BIRDS. 



521 



tone of these animals. Although the largest of the American 

 Monkeys, these Howlers are of inconsiderable size ; yet their 

 voices are louder than the roaring of lions, being distinctly 

 audible at the distance of two miles ; and, when a number are 

 congregated together, the effect is terrific. 



685. In Birds, the situation of the vocal organ is very 

 different. The trachea opens into the pharynx, as in Reptiles, 

 by a mere slit ; the borders of which have no other movement 

 than that of approaching one another, so as to close the aper- 

 ture when necessary. But at the lower extremity of the 

 trachea, just where it subdivides into the bronchial tubes, 

 there is a sort of larynx or vocal organ, which is of very 

 complex construction, especially in the singing-birds. The 

 external surface of this larynx is represented in fig. 264; its 

 muscles, m m', being left i 



in their places on one side, 



and removed on the other. , 



At 1 1, is seen the trachea ; 



at the lower extremity of 



which, ^, is a sort of bony * 



drum, I, divided at its , 



lower part by a partition of 



the same material (o, fig. b 



265), which is surmounted l 



by a semilunar membrane 



(c). This drum communi- c 



cates below with the two Fig. ZBI.-LAHYHX OF 



bronchial tubes, b b' (fig. 



264), each of which has its own glottis and vocal cords; 



the inner lip of one of these is seen at a (fig. 265) ; and at me 



is shown a drum-like membrane, forming the inner wall of 



the bronchial tube, which probably increases the resonance 



of the voice. These parts are acted-on by several muscles, 



the number of which varies according to the compass and 



flexibility of the voice in the different species ; being very 



considerable in the most esteemed of the singing-birds, and 



being reduced to a small amount in those which have no vocal 



powers. Iii some, indeed, they are altogether absent; and 



the state of the glottis can be influenced only by those muscles 



which raise and lower the whole trachea. 



686. The vocal sounds produced by the action of the larynx 



Fig. 265. VERTICAI 

 SECTION OF SAME. 



