400 



COMPARATIVE ZOOLOGY 



tions 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 158 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 bronchi. 159 

 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 pass- 

 ing 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 differences in the blast of air, as well as by changes 

 in the tension of the membrane. The range of notes 

 is commonly within an octave. Birds of the same 

 family have a similar 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. 160 The parrot and mock- 

 ing bird use the tongue in imitating human sounds. 

 Some species possess great compass 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 former 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 

 cartilages, of which the largest (the thyroid) produces the 

 prominence in the human throat known as " Adam's 

 apple," and two elastic bands, called " vocal cords," 

 just below the glottis, or upper opening of the wind- 

 pipe. The various tones are determined by the tension 

 of these cords, which is effected by the raising or lower- 



