444 AVES. 



of the membrana tympaniformis interna. In many cases a 

 membranous fold — the memhrana tympaniformis externa — is 

 developed on the external side of the tympanum, and forms with 

 the free edge of the internal tympaniform membrane (i.e. with 

 the membrana semilunaris), a vocal slit or glottis on either side. 

 The tension of these folds, which function as vocal chords, is 

 regulated by a muscular apparatus, which connects the trachea 

 with the lateral parts of the tympanum, or also with the an- 

 terior bronchial rings, and is most highly developed in the singing 

 birds, in which the syrinx may possess five or six pairs of such 

 muscles. This is the usual form of syrinx. It is occasionally 

 placed at the lower end of the trachea or at the upper end of 

 the bronchi ; in such cases its structure though essentially as 

 described presents some modifications. 



The bronchi are relatively short and lead, at their entrance 

 into the lungs, into a number of wide membranous bronchial 

 tubes, which traverse the pulmonary tissue. The lungs which 

 are of relatively small bulk are not, as in mammals, freely sus- 

 jDended in a closed thoracic cavity and invested by a pleural sac, 

 but are attached to the dorsal wall of the body cavity by cellular 

 tissue, and sank in the interspaces between the ribs at the sides 

 of the vertebral column. The behaviour of the bronchial tubes 

 and the structure of the finer resjoiratory air-spaces of the lungs 

 present essential differences from those of the Mammalia. The 

 air-sacs are expansions of the blind ends of some of the main 

 bronchial tubes ; they have thin, somewhat stiff membranous 

 walls, and project for the most part into the general body-cavity. 

 There are five pairs of them, called, in order from before back- 

 wards, the cervical (prebronchial), the interclavicular (infra- 

 bronchial), the anterior thoracic (anterior intermediate), the 

 j)Osterior thoracic (posterior intermediate) and the abdominal 

 (posterior). The main bronchus enters the lung of its side on 

 the ventral surface a short distance from the anterior end. It 

 is continued as the mesobrcnchium through the lung to its hind 

 end where it dilates into the large abdominal air-sac (Fig. 245, 

 La) which lies along the dorsal wall of the body-cavity in the 

 pelvic region on the ventral side of the kidneys, to which and to 

 the adjacent walls of the pelvis its dorsal wall is adherent 

 ventrally it is covered by peritoneum and is in contact 

 with the coils of the intestine. 



