THE INTERNAL SKELETON. 177 
may expand more or less from above downwards at their ends, 
becoming compressed from before backwards, where they join 
the sternum, as is well seen in the Ostrich, where each such 
elongated articular surface presents two superimposed articular 
cavities for its junction with the breast-bone. The sternal ribs 
increase in length from before backwards. 
The Sternum.—This is a very variously modified and charac- 
teristic part of a bird’s skeleton, the several forms it assumes 
helping to define difterent groups of birds. Its great size and 
the prominent median ridge or “keel” (which must be known 
to everyone who has carved a fowl) stand in close relation to 
flight, since the principal use of its great size is, as we shall see 
later, to provide sufficient space for the insertion of the muscles 
which both raise and depress the wing. In some instances, as 
in the Swan, the keel is much expanded and hollow and the 
windpipe makes a coil within it. The sternum of a bird 
answers to much more than our breast-bone, but might be 
represented in us by an imaginary extension of our sternum 
into a great sheet of bone passing downwards beneath part of 
the muscles of the abdomen or belly. It is a large continuous 
bony structure more or less convex ventrally both transversely 
and from behind forwards. 
The Bird’s sternum consists, in fact, of :—(1) An anterior 
part, into the sides of which are set the sternal ribs while its 
front margin affords a firm implantation to two bones—called 
** coracoids ”’—which mainly serve to support, as two fulcra, 
the anterior or thoracic limbs; and (2) a posterior portion 
which may be variously formed as follows :—It may be very 
short and broad, as in the Apteryx. Its posterior margin may 
be entire and obtuse, as in the Emeu; or entire and acutely 
prolonged, as in the Cassowary 3 ; or entire except that it has a 
short median notch, as in the Rhea; or with a short median 
prominence and two lateral ones, as in the Ostrich; or with a 
very long median one with two lateral notches on either side of 
it, as in the Common Fowl; or its posterior margin may be 
transversely continuous, while a little in front of it there may 
be two vacuities side by side, or five in a transverse series. 
Each such vacuity is called a fenestra or a fontanelle, 
A sternum which has neither notches nor fenestre is called 
entire, and, as just said, it may be single-notched or double- 
notched, or it may be wnifenestrate or bifenestrate, 
In the overwhelming majority of birds there is a keel, whence 
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