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 
ernal surface of this larynx is represented in fig. 264 ; its 
muscles, m m’, being left # 
‘in their places on one side, 
and removed on the other. + __& 
At ¢t, is seen the trachea ; 
at the lower extremity of | 
hich, ¢, is a sort of bony ‘ A 
drum Z, divided at its pee: e : 
lower part by a partition of 
he same material (0, fig. » . Ese 
265), which issurmounted % ~~¢: 
by a semilunar membrane 3 
This drum communi- , .& 
sates below with the two Fig. 26¢.—Larynx or Fig. 265.—Vertican 
bronchial tubes, b (fig. A Rook. SECTION 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 
fis 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 
Hexibility 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 
gowers. In some, indeed, they are altogether absent; and 
the state of the glottis can be influenced only by those muscles 
hich raise and lower the whole trachea. 
| 686. The vocal sounds produced by the action of the larynx 
