Mr. YAnRELL on the Organs of Voice in Birds. 315 
The first pair of muscles after passing down the sides of the 
tube in the ordinary way do not quit it to be attached to the 
sternum, but have their tendons inserted upon the outside of 
the second pair of muscles, their use being to influence the 
length of the tube, and assist the second pair in their action 
upon the bronchie (Tas. XVIII. Fig. 7, d). The second pair 
of muscles have their origin one on each side a little above the 
bones of divarication. Somewhat broad at first, they become. 
- narrower as they pass downwards, and are inserted upon the 
outer and central portion of the bronchiz at the fourth semi- 
circular cartilage (Fig. 7, e). The third pair of muscles arise 
broad and thick from the sides of the last ring of the tube, and 
are inserted over the whole surface of the moveable crescent- 
shaped bones below (Fig. 7 & 8, f), their obvious use being 
that of separating the inferior edges of these bones, thereby 
enlarging the aperture. Fig.8. represents a side view of the 
lower portion of the trachea in the Macaw, the three muscles 
detached from each other to render them more distinct. It may 
be necessary here to remark that the two sides of a trachea and 
their attendant muscles are invariably alike, except in some of 
the Anatide. Fig. 6. & 7. are two views of the same portion of 
the trachea with the three pair of muscles in situ. Their diffe- 
rent powers will be obvious on inspection ; the second pair, 
marked e, performing by their contraction the double office of 
altering the length of the bronchize, and, pressing upon the pro- 
jecting surface of the third pair of muscles, which they pass over j 
diminish the aperture formed by the edges of the moveable semi 
lunar-shaped side bones. It is to this power of diminishing the 
aperture these birds are indebted for the notes they are able to 
produce so high in the scale of tone. : 
I have never yet been fortunate enough to meet witha bird pos- 7 
sessing four pair of true muscles of voice. I proc therefor 
