LARYNX OP 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 Beptiles, 
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 rri ', being left t 
in their places on one side, 
and removed on the other, t .fc 
At 11 , is seen the trachea ; | % Si 
at the lower extremity of | J \—' m |||| 
which, is a sort of bony v .Ail 1 *.P™ 
drum, l , divided at its i 
lower part by a partition of 
the same material (o, fig. v ... r — f . . 
265), which is surmounted * 
by a semilunar membrane 
(c). This drum communi- c . 
cates below with the two Fig- 264.- Larykx of Fig. 265.— Vertical 
bronchial tubes, b V (fig. A Ro0K ' Seciion of same ’ 
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. In 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 
