112 AVES. 



no inferior larynx. It is true that in the Vultur cinereus and fulvus 

 as also in Gypaetos, there can not be found, as in many other Birds, 

 any coalescence of the lower tracheal rings, or an external tympani- 

 form membrane, but between the demi-rings of the bronchi (which 

 appear, however, in the American Vulture or Gallinazo to be almost 

 complete) there is situated the internal tympaniform membrane, 

 while a single pair of muscles placed externally at the extremity of 

 the trachea, serves to elevate slightly the bronchi upon either side, 

 shorten the trachea, and thus expand the two bronchio-glottidean 

 fissures. This pair of muscles acts moreover as an antagonist to the 

 sterno-tracheales. 



In the majority of those Birds which are capable of uttering 

 sounds membranes are found situated both exteriorly and internally 

 to the inferior larynx. There arises from the cross-bone a thin mem- 

 branous slightly elastic and easily lacerable membrane, which com- 

 pletes the bronchi upon their internal aspect. The extent of this 

 membrane is greater or less in proportion to that of the segment 

 formed by the imperfect bronchial rings ; the first two or three of 

 these are usually very slightly curved, and in the form only of a 

 small semicircle ; the membrane completing the rings is therefore 

 largest in this situation, and fully merits its appellation of membrana 

 tympaniformis internet. In some Birds, as in several Ducks, espe- 

 cially the Mergansers, large flat cartilaginous discs are situated in 

 this tympanic membrane, or, as in Fulica, thick cordiform cushions 

 of cellular tissue, either of which structures must exert an indubitable 

 influence upon the formation of the voice. There is generally found 

 a membrana tympaniformis externa presenting the form of a fenes- 

 troid oval membrane placed between the cross-bone or the lowermost 

 tracheal ring and the most superior of the bronchial semicircular 

 cartilages. 



This external fenestra or membrane may be wanting and stilt the 

 inner one present and the trachea form an osseous drum, as in the 

 Ducks and Mergansers. Or else the hard rings of the trachea may 

 lie closely approximated and invested by fibro-cartilage, beneath 

 which is situated the external ovale membrane, as in the Flamingo. 

 No important change however can be effected in the relative degree 

 of tension of the two membranes in cases where a single pair of 

 muscles is present, and attached high up to the border of a drum 

 formed of immoveable rings, whereas when inserted in the upper- 

 most bronchial rings, a far greater share of mobility is attainable by 

 the membranes. 



