AMERICAN SWAN. 679 



is much flattened, until about six inches from the furcula, 

 when it gradually becomes cylindrical, and seven-twelfths in 

 diameter, on reaching the curve of the furcula bends a little 

 upwards, enters a cavity formed in the crest of the sternum, 

 along which it passes to the length of six inches, bends upon 

 itself horizontally, forming a loop, returns, emerges, bends 

 upwards and backwards, entering the thorax. The inferior 

 larynx, composed of several united rings, is laterally com- 

 pressed. Appended to the last or semilunar ring, on each 

 side, is a narrow membrane, terminated by an extremely 

 slender half ring. Within this small membrane, and separate 

 from it, is a large membrane extending to the first bronchial 

 ring. The bronchi are very short, at first compressed, then 

 enlarged into a roundish cavity seven-twelfths-and-a-half in 

 diameter, afterwards cylindrical ; their entire length an inch 

 and three fourths ; the right bronchus with twenty-three, the 

 left with twenty-one slender rings, a few of which anastomose. 

 Length to end of tail 50 inches ; bill along the ridge 4£, 

 from its tip to the eye 4-j-£ ; lower mandible, along the edge 

 3^-, its height at the basal angles of the mouth 1 T 9 T , breadth 

 at the nostrils 1-^, near the end 1^-. 



Variations. — In the adult state, the principal variations 

 are in size, and in the extent of the orange-red, or rusty tint 

 on the head and neck. According to Dr. Sharpless, the 

 trachea in young birds forms a vertical fold within the keel 

 of the sternum, but in those of more advanced age, in which 

 a cavity forms in the body of the sternum, makes a horizontal 

 loop, which varies in size, until in very old birds it has a 

 width of two inches or more. Of three sterna figured by him 

 to show these gradations, one, in which the trachea has only 

 a vertical fold — the bird being very young — the length of the 

 sternum was six inches and a half, the length to which the 

 tracheal loop extended three inches and a half; in another, 

 the length of the sternum was six inches and a half, that ot 

 the loop, horizontal at the end, four inches and three fourths ; 

 in the third the sternum was seven inches and a half in 

 length, the loop circularly expanded at the end, and extend- 

 ing to the posterior edge of the sternum. 



