MECHANISM OF THE TUBE OF THE TRACHEA. 37 



one side, they are complete in the Grebes, in which moreover 

 the rings are ossified. As to the extensibihty and contractihty 

 of the trachea, I may state that the windpipe of a Golden Eagle, 

 when contracted to the utmost by the overlapping of its rings, 

 measured five inches and a half, but when extended to the ut- 

 most, measured nine inches and a half; that of a Curlew was 

 four inches long when contracted, and six inches and three 

 quarters when extended ; and that of a Cormorant, in the for- 

 mer case, nine inches and three quarters, and in the latter 

 fifteen inches and a quarter. But as in these cases the bones 

 of the upper larynx and syrinx, which do not slip over each 

 other, are included, a better idea of the extensibility of the 

 trachea is obtained from a fragment of it taken from the middle. 

 Thus, a portion of the trachea of a Rook, one inch long when 

 contracted as much as it can be, measures two inches and three 

 quarters when extended to the utmost ; and a portion of that 

 of a Wood Pigeon, one inch long when contracted, may be 

 extended to two inches and a quarter. But the greatest range 

 known to me is exhibited by the dilated portion of the trachea 

 of the Golden-eyed Duck, which may be contracted to a quarter 

 of an inch, and extended to two inches and a quarter. This, 

 however, is effected by a mechanism different from that usually 

 exhibited ; for although the rings cross each other in front, in 

 the ordinary manner, they are narrowed behind, and gradually 

 fall within each other upwards. 



Finally, I have again to remark that I do not consider the 

 pair of muscles, PL X, Fig. % ee\ PI. XI, Fig. 16, «?; PL 

 XII, Fig. 1, g g -^ which connect the trachea with the sternum, 

 as belonging to the lower larynx, with which they generally 

 have no connexion. They usually form the lower extremity of 

 the contractor-trachese or sterno-trachealis, but are sometimes 

 distinct, at least in part, some of their fibres being inserted upon 

 one or two rings of the trachea. 



And now, good Pupil, persevering and industrious, I cannot 

 suppose that you hold all this to be a digression. Many per- 

 sons consider everything anatomical, mathematical, or in any 

 way rational, as " dry :" they relish romances, rhymes, carica- 

 tures, and frivolities of all sorts. Some weak-headed naturalists 



