THE RESPIRATORY ORGANS IN SAUROPSIDA. 269 



complete internally, and have llie form of arches embracing 

 the outer moiety of the bnjnchus. The second and third of 

 these bronchial arcs are freely movable, and elastic tissue, 

 accumulated upon their inner surfaces, gives rise to a fold of 

 the mucous membrane, which forms the outer boundary of a 

 cleft, bounded, on the inner side, by the membrana semiluna- 

 ris. The air forced through these two clefts from the lungs 

 sets their elastic margins vibrating, and thus gives rise to a 

 musical note, the character of which is chiefly determined by 

 the tension of the elastic margins and the length of the tra- 

 cheal column of air. The muscles, by the contraction of which 

 these two factors of the voice are modified, are extrinsic and 

 intrinsic. The former are possessed by birds in general, and are 

 usually two pair, passing from the trachea to the furcula and to 

 the sternum. Some birds possessing a bronoho-tracheal syrinx 

 such as has been described, &s'ihe Alectromorphw, Ghenomorphce 

 and Dysporomorphm, have no intrinsic muscles. Most others 

 have one pair, attached, one on each side, to the rings of the 

 trachea above, and to the tympanum, or the proximal bronchial 

 arcs, below. The majority of the Goraconiorphoe, have five or 

 six pairs of intrinsic syringeal muscles, which pass from the 

 trachna and its tympanum to the movable bronchial arcs. The 

 Parrots have no septum, and only three pairs of intrinsic 

 musel es. 



T^ie tracheal syrinx occurs only in some American Cora- 

 comophce. The hinder end of the trachea is flattened, and 

 six or seven of its rings above the last are interrupted at the 

 sides, and held together by a longitudinal ligamentous band. 

 Thes' rings are excessively delicate, so that this part of the 

 trach';a is in great part membranous. 



T<ie hronchial syrinx occurs only in Steatornis, and Gro- 

 tophaga. 



I'l the genus Ginyxis, among the Ghelonia, and in some 

 species of Grocodilus (C acMiws, e. g.), the trachea is bent 

 upon itself. Similar flexures attain an extraordinary develop- 

 ment in many birds, and may lie outside the thorax under the 

 integument ( Tetrao urogallus, some species of Grax and Pe- 

 7ielope) ; in the cavity of the thorax (some Spoonbills) ; on the 

 exterior of the sternum (some Swans and Cranes) ; or even 

 in a sort of cup formed by the median process of the furcula 

 (the Guinea-fowl). In the Eraeu some of the "rings of the 

 trachea are incomplete in front, and bound tlie aperture of an 

 air-sac which lies in front of the trachea. Some birds {Apte- 

 nodytes Procellaria) have the trachea divided by a longitu- 



