58 



STRUCTUKE AND CLASSIFICATION OF BIRDS 



Respiratory System 



Trachea. — As a general rule the trachea is a straight tube 

 passing into the thorax, where it bifurcates into the two 

 bronchi. It is composed throughout of rings, which are 



cartilaginous, or may be 

 wholly or partially ossified. 

 The rings are generally 

 simple rings, which are like 

 each other and quite com- 

 plete, excepting just at the 

 bifurcation, where it is com- 

 mon for them to be modi- 

 fied in connection with the 

 formation of the syrinx. 



This is especially marked 

 in the tracheophone Passeres 

 and in the Ciconiidse, to the 

 accounts of which families (as 

 well as to below, ' Syrinx ') 

 reference must be made for 

 the facts. Other modifica- 

 tions of some of the last 

 tracheal rings are to be seen 

 in the cassowary, where the 

 last few are incomplete 

 behind, as in the mammals 

 and in the bird of Paradise, 

 Seleucides (see fig. 38), where 

 the membranous interspaces 

 between the rings become 

 largely increased, and the 

 rings ossified at both sideSj 



Fig. 38. — Windpipe op Seleucides nigra. 



4-11, tracheal rings ; III, third bronchial ; 5.(, 

 sterao-tracheal muscles. (After Fouees.) 



but not in the middle, in a peculiar fashion. 



A peculiarity of the trachea, seen in representatives of 

 some most diverse groups of birds, is its looping. This is, 

 of course, suggestive of the similar looping of the trachea in 



