214 



BliEMENTS OF OENITHOLOGT. 



additional muscles are present in Singing-birds ; but also in 

 many Birds which do not sing, as, e. g., the Crow and Eaven. 



There may be no " membrana semilunaris," and only three 

 pairs of additional muscles, in birds highly gifted as to their 

 power of emitting special sounds. A syrinx may be formed by 

 the trachea only (without the intervention of the bronchi), as in 

 Thamnophilus, where the lower part of the trachea has delicate 

 walls, and is flattened dorso-ventrally into six or seven delicate 

 segments of rings, the rings being interrupted laterally. 



A syrinx may be formed in each bronchus (without the inter- 

 vention of the trachea), as in Steatomis, where more than ten 

 rings in each bronchus may be counted before reaching the 

 syrinx, and where a pair of muscles pass from each bronchus 

 to the trachea. 



Fig. 165. 



Eight Lung of a Goose (after Owen). 



a, Bronchus ; b, b, openings into air-sacs. (In the two bronchi which are cut 

 open are seen the apertures of their primary branches.) 



The intrinsic muscles of the voice-organ may be inserted into 

 the ends of the bronchial semi-rings, or in what is called an 

 Acromyoidal manner, or into their middle parts, a mode which 

 is distinguished as Mesomyoidal. A condition in which the 

 trachea alone forms the vocal organ is spoken of as " Tracheo- 

 phonal." An arrangement in which the lower end of the trachea 

 is not modified to form a vocal organ is called Oligomyoidal. 



Parrots have no os transversale or septum dividing the lower 

 end of the trachea, and they have only three pairs of intrinsic 



