84 



SPECIAL SENSES. 



180. In man and the mammals, the voice is formed in an 

 Fig. 59. organ called the larynx, situated at the upper 

 part of the windpipe, below the bone of the 

 tongue (a). The human larynx, the part called 

 Adam's apple, is composed of several cartilagi- 

 nous pieces, called the thyroid cartilage (6), the 

 cricoid cartilage (c), and the small arytenoid car- 

 tilages. Within these are found two large folds 

 of elastic substance, known by the name of the 

 vocal cords (m). Two other analogous folds, 

 the superior ligaments of the glottis (n), are situated a little 

 above the preceding. The glottis (o) is the space between 

 these four folds. The arrangement of the vocal cords, and of 

 the interior of the glottis in man, is indicated by dotted lines 

 in fig. 59. 



181. The mechanism of the voice is as follows : the air, on its 

 way to the lungs, passes the vocal cords. So long as these are in 

 repose, no sound is produced ; but the moment they are made 

 tense, they narrow the aperture, and oppose 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 ten- 

 sion is great, and a grave and dull 

 one when the tension is feeble. 



182. Some mammals have, in ad- 

 dition, large cavities which commu- 

 nicate with the glottis, and into which 

 the air reverberates, as it passes the 

 larynx. This arrangement is espe- 

 cially remarkable in the howling mon- 

 keys, which are distinguished above 

 all other animals, for their deafening 

 howls. 



183. 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 infe- 

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

 kind of 'bony drum (fig. 60 a), having within it two glottides, 



