20 VOCAL MACHINERY. 



into their anatomical structure, describes the trachea as an 

 elastic tube, extremely flexible and contractile, covered 

 with layers of cellular tissue, and accommodating itself to 

 all the motions of the neck. It commences behind the 

 tongue, extending to opposite the first rib, where at the 

 syrinx, as it is called, or lower larynx, it divides into two 

 bronchii or branches. In man and in quadrupeds, the 

 parts are proportionably larger and more complex ; the 

 voice, with all its varieties of tone, being produced by the 

 muscles and cords of the larynx ; while in birds it is pro- 

 duced at the lower extremity of the syrinx, or windpipe, 

 and modulated into notes by the contraction and extension 

 of the larynx. In the human larynx, the vocal chords, 

 which vibrate under the impulse of the air, and thus 

 produce wind, are placed in the larynx; but in birds 

 there are no traces of them there, the vibrating membrane 

 being placed in the syrinx, or lower larynx. Such is the 

 apparatus by which the voice of birds is attuned. The 

 air contained in the lungs and air cells, passing through 

 the bronchii, causes the vocal membranes at the anterior 

 extremity to vibrate, and thus produce sound, which is 

 rendered grave or acute by the relaxation or tension of the 

 parts ; and the stream of air thus thrown into vibration 

 is divided, narrowed, or suffered to pass free, by the 

 muscles of the larynx. i The modification of these organs 

 presented by the different species,' says the naturalist, 

 'are slight, the parts in all I have examined being the 

 same, and each the same number of muscles. The peculiar 

 song of different species must therefore depend on circum- 

 stances beyond our cognition. For surely no one could 

 imagine the reason that the Kook and the Hooded Crow 

 require as complex an apparatus to produce their unmusical 

 cries, as that which the Blackbird and Nightingale employ 

 in modulating their voices, so as to give rise to those 

 melodies which are so delightful to us ; and yet the knife, 

 the needle, and the lens do not enable us to detect any 

 superior organisation in the Warbler over the Crow.' 



This is another of those mysteries which constantly 

 puzzle and perplex the natural philosopher ; like the diver- 

 sities in the forms and habits of every creature, like the 



