THE NERVOUS SYSTEM. 
189 
are caused, in part, by the vibrations of the wings; but 
the true voice of these Insects comes from the spiracles 
of the thorax. 
Snakes and Lizards have no vocal cords, and can only 
hiss. Frogs croak 104 and Crocodiles roar, and the huge 
Tortoise of the Galapagos Islands utters a hoarse, bellow¬ 
ing noise. 
The vocal apparatus in Birds is situated at the lower 
end of the trachea, where it divides into the two bron¬ 
chi. 105 It consists mainly of a bony drum, with a cross- 
bone, having a vertical membrane attached to its upper 
edge. The membrane is put in motion by currents of air 
passing on either side of it. Five pairs of muscles (in the 
Songsters) adjust the length of the windpipe to the pitch 
of the glottis. The various notes are produced by differ¬ 
ences in the blast of air, as well as by changes in the ten¬ 
sion of the membrane. The range of notes is commonly 
within an octave. Birds of the same family have a simi¬ 
lar voice. All the Parrots have a harsh utterance; Geese 
and Ducks quack; Crows, Magpies, and Jays caw; while 
the Warblers differ in the quality, rather than the kind, of 
note. 108 The Parrot and Mocking-bird use the tongue in 
imitating human sounds. Some species possess great com¬ 
pass of voice. The Bell-bird can be heard nearly three 
miles; and Livingstone said he could distinguish the voices 
of the Ostrich and the Lion only by knowing that the for¬ 
mer roars by day, and the latter by night. 
The vocal organ of Mammals, unlike that of Birds, is 
in the upper part of the larynx. It consists of four car¬ 
tilages, of which the largest (the thyroid) produces the 
prominence in the human throat known as “Adam’s ap¬ 
ple,” and two elastic bands, called “ vocal cords,” just be¬ 
low the glottis, or upper opening of the windpipe. The 
various tones are determined by the tension of these cords, 
which is effected by the raising or lowering of the thyroid 
