PECULIAR MECHANISM IN THE TRACHEA OF BIRDS. 239 



has been already observed by others. In Carus's Traite d' Anatomie Comparee, 

 it is stated that " not unfrequently, for example in the Heron and Swan, the 

 left half and the right half of the rings of the trachea are alternately broader, 

 which produces an appearance somewhat of this nature before and behind" (See 

 Plate, fig. 1.). Mr. Owen also says,— " they are generally of uniform breadth, 

 but in some species are alternately narrower at certain parts of their circum- 

 ference, and broader at others, and in these cases the rings are generally closely 

 approximated together, and, as it were, locked into one another. This structure 

 is most common in the Grallatores, where the rings are broadest alternately on 

 the right and left sides : the French Academicians have given a good illustration 

 of this structure from the trachea of the Demoiselle Crane." I mention these 

 authors because they are the only ones at hand, and because I hold them in 

 great estimation ; but, nevertheless, I am obliged to assert that they and the 

 " French Academicians" have been deceived. Not having examined the tracheas 

 of all the birds in the world, I cannot, of course, say that such a structure does 

 not exist ; but I have not met with it in anv bird hitherto examined by me, and 

 am certain that it is not to be found in the Cranes, Herons, or Swans, as alleged. 

 The real state of the matter is this : — 



Owing to the frequent and extensive alternate contractions and elongations 

 of the neck, the trachea requires to have a structure allowing it to undergo 

 corresponding alterations, and this without any great change in its diameter. 

 Solid rings connected by elastic membrane might, by the contraction of the latter, 

 approximate so closely as greatly, to diminish the length of the tube ; but so 

 great does this diminution occasionally require to be, that, to effect it in this 

 manner, the rings would be too slender or too distant to maintain the calibre 

 of the tube in a perfectly pervious state, and therefore a contrivance was neces- 

 sary by which strength and a great degree of contraction, with a uniform 

 diameter in all cases, might be combined ; and this has been effected in the 

 following manner. 



Fig. 2 in the plate represents a portion of a windpipe in a state of relaxation ; 

 while Fig. 3 represents a portion in a state of contraction. In the former state, 

 Fig. 2, the rings, which are equal, or nearly so, stand free, being separated by an 

 intervening space occupied by elastic membrane. In the latter state, Fig. 3, the 

 rings appear as if incomplete, or the trachea seems to be formed of alternate 

 lateral half rings, presenting at their meeting in front and behind a zigzag line. 

 In other cases, as in Fig. 4, 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 half narrow. And it is thus that the rings of the trachea 

 in many birds have been usually represented. But let us examine the matter 



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