£40 
PECULIAR MECHANISM IN THE TRACHEA OF BIRDS. 
more closely; 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. 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, inches; that of a Curlew was 4 inches long when 
