39 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



for the male to be much better provided with them 

 than the female. 



Sound-producing organs are much less common 

 among butterflies and moths, and where present, 

 they seem to be due to the vibration of a membrane, 

 and not to the movement of a rasping organ, as in 

 beetles. 



Among the Vertebrata, voice, or definite and more 

 or less musical sounds, are ordinarily produced by the 

 vibration of the column of air which passes down the 

 trachea and sets in movement the membranes (vocal 

 membranes), which lie on either side of that por- 

 tion of the trachea which is distinguished as the 

 larynx ; they are supported by definite cartilaginous 

 pieces (arytenoid cartilages), and bound a narrow 

 cleft which is known as the glottis. While this 

 simple condition is, for example, found, among the 

 Amphibia, in some frogs, others have well-developed 

 sacs connected with the larynx, which become swollen 

 out and project on either side of the head ; these sacs, 

 which are often better developed in males than females, 

 take an important share in increasing the noise made 

 by their possessor, which may sometimes be heard at 

 a great distance. 



Among Reptiles, where the laryngeal apparatus is, 

 on the whole, comparatively simple, the chamseleons 

 are provided with air sacs, which do not appear on 

 the surface of the animal as they do in the edible 

 and some other, though not all, frogs. 



Birds are remarkable for the fact that their vocal 

 organ is not, as in other Vertebrates, formed at the 

 top, but a't the bottom of their trachea, and at the 

 point where the trachea divides into the two bronchi ; 

 the syrinx, as the organ of voice in birds is called, is 

 best developed in the Passeres, where a share in its 

 formation is taken both by the trachea and by the 

 bronchi (foroiicliio-tracheal syrinx). 



