66 



OF THE VOICE. 



an obstacle to the current of air, and it cannot pass without 

 causing them to vibrate. These vibrations produce the 

 voice ; and as the vocal cords are susceptible of different 

 degrees of tension, these tensions determine different sounds ; 

 giving an acute tone when the tension is great, but a grave 

 and dull one when the tension is feeble. 



125. Some mammals have, in addition, large cavities 

 which communicate with the glottis, and into which the air 

 reverberates, as it passes the larynx. This arrangement is 

 especially remarkable in the howling monkeys, which are dis- 

 tinguished above all other animals for their deafening howls. 



126. In birds, the proper larynx is very simple, destitute 

 of vocal cords, and incapable of producing sounds ; but at 

 the lower end of the windpipe there is a second or inferior 

 larynx, which is very complicated in structure. It is a kind 



of bony drum, (a,) having with- 

 in it two glottides, formed at the 

 top of the two branches (bb) of 

 the windpipe, (c,) each provided 

 with two vocal cords. The dif- 

 ferent pieces of this apparatus 

 are moved by peculiar muscles, 

 the number of which varies in 

 different families. In birds which 

 have a very monotonous cry, 

 such as the gulls, the herons, 

 the cuckoos, and the mergansers, 



(Fig. 23,) there is but one or two pairs ; parrots have three ; 



and the birds of song have five. 



127. Man alone, of all the animal creation, has the power 

 of giving to the tones he utters a variety of definite or ar- 

 ticulate sounds ; in other words, he alone has the gift of 

 speech. 



Fig. 23. 



