MECHANISM OF THE TUBE OF THE TRACHEA. 35 



ing in front and behind, a zigzag line. In the other case, Fig. 

 110, the rings appear as if complete, but alternately broader on 

 either side ; in other words, each ring seems to have one lateral 

 half broad and the other narrow. And it is thus that the rings 

 of the trachea in birds, or at least in many birds, have been 

 usually represented. But let us examine the matter more closely, 

 and rather take a lesson from nature than from books ; and for 

 this purpose let us take the trachea of a large bird, a Wild 

 Swan for example. 



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

 when contracted to the utmost, Fig. Ill, appears to be com- 

 posed of alternating half rings ; or if 

 less contracted, of entire rings alter- 

 nately broad and narrow. But the 

 same portion drawn out to its full ex- 

 tent, and then measuring two inches 

 in length, Fig. 112, has a very diffe- 

 rent appearance, being evidently com- 

 posed of equal rings, each, however, 

 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 pecu- 

 liar manner, so as to produce the appearance represented by 

 Fig. 111. Fig. 113 represents two rings, the membrane be- 

 tween 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 riofht. In Figf. 



114, these two rings 

 are brought close toge- 

 ther, when it is seen 

 Fio. 113. Fig. 114. that No. 1 has over- 



lapped No. 2 on the right side as far as the ridge, while No. 2 

 has overlapped No. 1 on the left side to the same extent. No. 1 



Fig. lU. 



