240 PECULIAR MECHANISM IN THE TRACHEA OF BIRD?. 



more closely ; and, for this purpose, let us take the trachea of a large bird, 

 Wild Swan for example. 



A portion of the trachea of that bird, one inch in length, when contracted tc 

 the utmost, Fig. 5, appears to be composed of alternating half rings ; or, if less 

 contracted, of entire rings alternately broad and narrow. But the same portion 

 drawn out to its full extent, and then measuring two inches in length, Fig. 6, has 

 a very different appearance, being evidently composed of equal rings, each how- 

 ever having a contraction, or two opposite notches, in the middle, in front, as 

 well as behind. Now, if the rings be gradually brought together, we find that 

 one overlaps the other in a peculiar manner, so as to produce the appearance 

 represented by Fig. 5. Fig. 6 represents two rings, the membrane between 

 which has been removed. The ring marked 1, is seen to be narrowed in the 

 middle, and the portion of it to the right is plain or flat, while that to the left 

 has a slight ridge running along its centre. The ring marked 2, is similar, but 

 has the plain portion to the left, and that with the ridge placed to the right. 

 In Fig. 7, these two rings are brought close together, when it is seen that N. 1 

 has overlapped N. 2 on the right side, as far as the right, while N. 2 has over- 

 lapped N. 1 on the left side to the same extent. N. 1 has thus passed within 

 N. 2 on the left side, and over it to the right ; and in this manner has been 

 produced the appearance exhibited by Fig. 4. Without the contraction or double 

 notch in each ring, this effect could, of course, not be produced, for it is there 

 that the rings cross each other. 



Thus, then, the trachea in its extreme state of contraction, when it is reduced 

 one-half or one-third of its greatest length, undergoes but a very slight dimi- 

 nution in its diameter. The elastic membrane passes from the edge of one ring, 

 over the next, to be inserted into the edge of the third. The intermediate ring 

 thus slips in behind those on each side of it ; while its other lateral half slips 

 before those on each side. This mechanism I have observed in almost all the 

 tracheae which I have examined, although the rings vary much in breadth and 

 thickness in different species. 



In some birds the rings of the trachea are very narrow, and their intervals 

 proportionally wide ; and sometimes the rings are cartilaginous, or but partially 

 ossified. All these circumstances are observed in the trachea of the Golden 

 Eagle, the White-tailed Sea-eagle, the Peregrine Falcon, and the Grebes. Often, 

 on the other hand, as in Cranes, Herons, Swans, and Mergansers, the rings are 

 very broad, with very small intervals. As to the extensibility and contractility 

 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 5^ inches, but when 

 extended to the utmost, 9£ inches ; that of^Curlew was 4 inches long when 



