214 ELEMENTS OF ORNITHOLOGY. 
additional muscles are present in Singing-birds; but also in 
many Birds which do not sing, as, ¢.g., the Crow and Raven. 
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 bas 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 Steatornis, 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. 

Ricut Lune or A Goose (after Owen). 
a, Bronchus ; 3, 6, 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 “ Z’racheo- 
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 
