228 ON INSECT SOUNDS. 



membrane ; but as the number of air vibrations depends upon 

 the times of vibration of the membrane, the pitch of the note 

 given out depends upon the tension of the membrane. For the 

 time of vibration varies inversely with the length of the membrane,, 

 and directly as the square root of the extending force. Where, as 

 in the human larjnx, provision is made for varying the tension of 

 the membrane (by its attachment to moveable cartilages, and by 

 the action of tightening and relaxing muscles) the pitch of the 

 note can be so varied as to allow of a complete scale of sounds. 

 And similarly, slight variations of pitch are provided for in 

 certain insect organs, by arrangements for tightening the 

 membrane, and probably also for increasing the force of the air 

 current, which increases at the same time the tension of the 

 membrane. But in the hunan voice-organ a remarkable addition 

 to the larynx is made, which partly answers the purpose of the 

 sound pipe of a reed instrument such as is fitted to our musical 

 instruments (organ, clarionet, bassoon, &:c.,) but which is more im- 

 portant for the facilities which it offers for regulation of the voice 

 as in singing, and articulation as in speech. The larynx does not 

 open directly on the surface of the body as in insects, but is situate 

 below the cavities of the nose, mouth, and fauces, the several 

 parts of which are constructed of elastic and moveable tissues, 

 whilst the palate, tongue, cheeks, and lips are supplied with a 

 variety of voluntary muscles, whereby an infinite variety of com- 

 bined movements can be brought into play, and additional tones- 

 and modifications of tones produced. Thus the reed sound may 

 be entirely avoided, and by help of the cavities above the larynx, 

 a sound produced, as in whispering or muttering, which may be 

 rendered articulate by the movement of the lips and lower jaw, 

 or it can be turned into a hissing, by projecting the tip of the 

 tongue between the front teeth, and opening the lips. This same 

 hissing sound may issue even from the larynx, as in the case 

 of the serpent. 



The fundamental laryngeal or reed tone, nevertheless, constitutes 



