470 



ELEMENTAR Y ANA TOMY. 



[LESS. 



FIG. 396. SYRINX OF 



A RAVEN. 

 a, b, and c, three bony 



structures. 



agents of song. Their action is modified by special muscles. 



Generally there are two pairs of muscles passing from the 



clavicles or sternum to the trachea ; and in addition to these 

 may be five or six pairs of muscles pass- 

 ing down from the trachea to the bron- 

 chial rings. These additional muscles are 

 present in singing birds, but also in many 

 birds that do not sing, as e.g. the Raven. 



There may be no membrana semi- 

 lunaris, and only three pairs of additional 

 muscles, in birds highly gifted as to their 

 powers of emitting special sounds. Such 

 is the case in the Parrots, where, however, 

 the lateral elastic folds of membrane are 

 well developed. 



A syrinx may be formed by the trachea 

 only (without the intervention of the 

 bronchi), as in Thanmophilus and Opetio- 



rhynchus, where the post-axial part of the trachea has delicate 



walls, and is flattened dorso-ventrally with six or seven deli- 

 cate segments of rings, the rings being 



interrupted laterally. The additional 



muscles here pass from the part of the 



trachea above (pre-axial to) the syrinx 



to the tracheal ring beyond it. 



A syrinx may be formed in each 



bronchus (without the intervention of 



the trachea), as in Steatornis, where 



more than ten rings in each bronchus 



may be counted before reaching the 



syrinx, and where a pair of muscles 



passes to each bronchus from the 



trachea. 



Bronchi may be absent together with 



the trachea, as in the common Frog 



and Toad. They may be absent though 



a trachea is present, as in Siren. They 



may open at once into the lung, without 



ramifying, as in Ophidians. They may 



traverse the lung, soon losing their car- 

 tilaginous rings, and having apertures 



in each side leading into the large pouch-like air-cells of the 



lungs, as in the Crocodile. They may similarly enter the lungs 



and lose their rings, but give off secondary branches at right 



FIG. 397. RIGHT LUNG OF 



A GOOSE. 

 {After Owen.) 

 a, bronchus ; l>, b, openings 

 into air-sacs. In the two 

 bronchi which are cut open 

 are seen the apertures of 

 their primary branches. 



