RAVEN. 271 



the vocal organ in Birds, is at the bottom of the tube, and 

 is formed by the more or less firm union of several of the 

 lower bony rings of the trachea, as shewn in Fig. 5 (b), 

 where the parts are divested of their attachments, and again 

 in Fig. 6 (b) where one bronchus has also been removed to 

 exhibit the inner side (c) of the other, together with a 

 median cross-bone, as represented in Fig. 7 (a, a), extend- 

 ing from behind to the front, and dividing the tube into 

 two equal parts, from the outer side of each of which the 



Fig. 5. Fig. G. 



bronchi spring and diverge to the lungs*. From the upper 

 edge of this cross-bone a crescent-shaped membrane, concave 

 above, ascends for a short distance inside the main tube, and 

 thus forms a " three-way piece ". 



The bronchi are, like the trachea, perfectly flexible, but the 

 rings of which they are formed, though similarly connected 

 with one another by membrane, as shewn in Fig. 5 (c), are 

 incomplete on the inner side, which is composed of a 

 delicate membrane, known as the membrana tympan'tformis, 

 seen in Fig. 6 (c), on the change of form and length of 

 which some of the varieties of intonation depend. These 

 tubes diminish in size as they approach the lungs, and they 

 are slightly attached to each other and to the oesophagus. 



The muscles of the glottis consist, so far as is known, 

 uniformly of the two pairs already described, but those of 



* In one group of Birds — the American Vultures (Cathartidiv) — and therein 

 only, it is believed, there is no special modification of the trachea into a syrinx. 

 VOL. II. N N 



