42 



OF THE VOICE. 



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 (Z>&) 

 of the windpipe (c), each 

 provided with two vocal cords. 

 The different pieces of this ap- 

 Iparatus are moved by peculiar 

 muscles, the number of which 

 varies in different families. In 

 birds which have a very mono- 

 |^l tonous 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 sounds ; 

 in other words, he alone has the gift of speech. 



Fig. 23. 



