RESPIRATORY SYSTEM OF BIRDS. 



221 



Tracheal coils and lower larynx, Bewick's Swan, xlviii- 



bronchi are incomplete. In the King-Vultnre the entire rings 

 are continued a little way along the bronchial divisions of the 

 trachea, without any modifications, external or internal, indicative 

 of a laryngeal structure. The same may be seen in the Ostrich, 

 where the bronchi are provided with entire slender rings rapidly 

 diminishing in size as they approach the lungs : but the terminal 

 rings of the trachea are thickened and protrude outward, forming 

 a cavity on each side, the 

 lining substance of which 

 projects into the area of the 

 tube above the commence- 

 ment of each bronchus.^ 



In most Birds the bron- 

 chi, figs. 85, V, 101, h, 

 are straight, compressed, 

 and easily lacerable tubes, 

 strengthened by half-rings 

 on the outer side, the inner 

 side being formed by a mem- 

 brane ('membrana tympani- 

 formis '). Usually the bronchi rapidly contract as they approach 

 the lungs. 



The muscles of the trachea are the ' sterno-tracheales,' fig. 104, 

 d, a long pair, arising from the costal processes of the sternum and 

 converging to ascend along the sides of the windpipe. To these 

 are sometimes added a second pair from the furculum, called 

 ' cleido-tracheales.' In Cursores and most Rasores the sterno- 

 tracheales alone are present. In most Raptores and Grallatores, a 

 muscle, broncho- trachealis, situated on each side of the lower part 

 of the trachea, descends to be inserted into the first or second 

 bronchial half-ring : in Alcedo and Caprimulgus it descends to the 

 third half-ring ; in some of the Owls its insertion is still lower, 

 and the degree of tension of the tympaniform membrane will be 

 proportionally varied. In Colopterus crisfatus an azygos muscle 

 occupies the anterior interspace of the broncho-tracheales.^ 



In other Yocal Birds there is a double glottis, usually produced 

 by a bony bar, *pessulus,' ' os transversale,' fig. 102, i, which 

 traverses the lower end of the trachea from before backward : it 

 supports a thin membrane which ascends into the tracheal area 

 and, terminating there by a free concave margin, is called the 

 ' membrana semilunaris,' ib. h. This is most developed in Singing 



sx. vol. ii. p. 103 (1834). 



- XLIX-. 



