PECULIAR MECHANISM IN THE TRACHEA OF BIRDS. 
239 
has been already observed by others. In Carus’s Traite d’ Anatomie Comparee y 
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 Gmllatores , 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 any 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 
