THE INTEBNAI, 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 different 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 Umbs; 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 ; 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. 



Asternum which has neither notches nor fenestrse is called 

 entire, and, as just said, it may be single-notched or doiible- 

 notclied, or it may be unifenestrate or bifenestrate. 



In the overwhelming majority of birds there is a keel, whence 



