Mi\ Yarrell on the Organs of Voice in Birds. 315 



The first pair of muscles after passing down the sides of the 

 tube in the ordinary w^ay 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 bronchiae (Tab. 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 bronchiae 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, /), 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 JnatidcE. 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 oflSce of 

 altering the length of the bronchiae, and, pressing upon the pro- 

 jecting surface of the third pair of muscles, which they pass over, 

 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 with a bird pos- 

 sessing four pair of true muscles of voice. I proceed therefore 

 « to 



