5G2 ORGANS OF RESPIRATION. 



muscles, the ypsilo- and sterno-tracheales, which are some- 

 times of great length. The tracheal differences of the sexes, 

 and of nearly allied species, affect, also, the inferior larynx, 

 and are greater in birds than in any other classes ; and they 

 are sometimes connected with differences of the voice. The 

 superficial retractor muscles are often wanting, especially in 

 wading birds, as in the long necked phcenicopterus, where 

 there are, at least, three hundred and fifty tracheal rings ; a 

 number not surpassed in any other bird. 



The inferior larynx of birds, like other vocal organs, is a 

 mere accessary development of the trachea, placed on that 

 tube to take advantage of the elasticity of the transmitted 

 air, for the purpose of producing sounds, and thereby giving 

 audible expression to the inward feelings. The ordinary and 

 most convenient seat of these sounds, the superior larynx, is 

 developed in birds, as in other air-breathing vertebrata, and 

 its vibratile margins can partially vocalize the transmitted air, 

 like the larynx of many reptiles, to which that of birds is 

 most closely allied in structure. But the great length, and 

 the varying dimensions of the trachea, the unsuitable form of 

 the tongue and buccal parietes of birds for modulating sounds, 

 and the demand for extensive vocal intercourse in this class, 

 necessitate the development of a new organ of voice, an in- 

 ferior larynx, placed nearer to the lungs and air-cavities. 

 This inferior or bronchial larynx, situated at the bifurcation 

 of the trachea, and involving the commencements of the two 

 bronchi, consists of enlarged, ossified, often much altered and 

 anchylosed, tracheal and bronchial rings, with great mem- 

 branous interspaces. The lowest tracheal ring, by extending 

 centrad before and behind, bisects its canal with an osseous 

 septum ; and the membranes of the interspace, by passing 

 centrad from each side, form two free internal crescentic vocal 

 ligaments. Two semilunar membranes also extend, in sing- 

 ing birds, from each side of the median septum, to meet the 

 lateral folds, and complete the vibratile lips of these two 

 rimae ; two simpler vocal folds, at the narrow membranous 

 origin of each bronchus, are likewise added to this complex 

 wind-instrument. There are five pairs of muscles for the 

 various movements of the inferior larynx in singing birds ; in 

 parrots, where this part of the vocal apparatus is more simple, 

 theree are thre pairs of muscles ; in many rapaceous, grallato- 



