112 The Convolution of the Trachea in the | February, 
From here the trachea reaches the point of entrance by a strong 
double curve, forming a figure which is a perfect letter S, the 
lower turn being within the first large coil, and the upper follow- 
ing the anterior outline of the keel. The cavity is three and one- 
quarter inches long, just one-half of the entire length of the keel. 
There are two unoccupied spaces between the coils—a small one 
filled with bony meshwork and a larger one entirely hollow. This 
description is from the specimen figured, which has been selected 
as representing very nearly the average of four. Two others 
show some variations worthy of mention. In a large sternum 
from a female bird, there is less convolution and fully two inches 
less of windpipe inside the keel. The upper loop does not fill the 
anterior part of the cavity, and on entering the keel the trachea 
does not go as far back or form the same kind of curve. But 
little more than one-half of the length of the keel is excavated. 
The third specimen shows a higher development than the one 
figured. The coils are large and occupy nearly the entire hollow, 
which is fully one-half the length of the keel. The posterior 
lower edge of the keel is thickened—much like the same part fur- 
ther forward—and cancellated inside; while the walls of the an- 
terior cavity are more fully ossified than in either of the others. 
In an embryo crane}! stated to be the sandhill, which was just 
about to break the shell when collected, the trachea does not enter 
the sternum at all and is perfectly simple. But the anterior part 
of the keel, which is, of course, entirely cartilaginous and very 
diminutive, is, comparatively speaking, much thickened, and a 
cross section shows it to consist of two thin walls separated by 
a marrow-like substance. In this feature of the sternum, we see 
the only indication in the embryo of the singular structure to be 
developed later in life. The degree of complexity of the trachea 
is thus shown to be dependent upon age, and the variations just 
alluded to are no doubt fully accounted for by this fact. ' 
It is unnecessary to frame a new description of the convolu- 
tions of the trachea in americana, as that furnished by Dr. Coues is 
excellent. The following extracts from it are given for the con- 
venience of the reader. “The sternal keel is broad and tumid, 
and is entirely excavated. The greater part of the ances is 
occupied by the singular duplications of the trachea; * but 
1 This specimen was obtained in the northwestern part of Minnesota, by Mr. Na- 
than Butler, and is now in the collection of the Minn. Acad. of Nat. Sci., Minne- 
sane Minn. ut 
we 
