AIR-TUBES AND LARYNX 



369 



dilator and one or more constrictors of the glottis. A differentiation 

 of these into distinct pharyngeal and laryngeal muscles occurs 

 only in higher forms. 



Reptiles. The cartilaginous rings of the trachea gradually 

 become more solid and complete in Reptiles, and as in other 



Fin. 276. SECTION THROUGH THE SYRINX OF A MALE BLACKBIRD (Turdus 



menila). (After V. Hacker. ) 



Id, brochidesnius ; B.I III, 1st- 3rd bronchial ring ; h, ventral cavity (part 

 of the anterior thoracic air-sac) ; l.e and /./, external and internal labia ; M, 

 muscles; m.t.e and m.t.i, membrana tyinpaniformis externa and interna ; si, 

 membrana semilunaris ; St, pessulus ; T, "tympanum"; T.r, tracheal rings. 



Amniota are well developed, though not always complete dorsally. 

 The length of the windpipe varies according to the length of the 

 neck and to the point at which it bifurcates into the two bronchi, 

 cartilages being present in these also, and gradually extending 

 along their intrapulmonary portions. 



B B 



