1012 ON FORM AND MECHANICAL EFFICIENCY [ch 



on either side, as in Fig. 485, will serve as a model of the skeletal 

 cantilever and shew how its limp surface is stiffened by the 

 folds. 



Save in the ostrich, and a few other flightless birds, the breast-bone 

 or sternum is a broad, flat bone, produced into a deep, descending 

 ridge or "keel," Very .firmly fixed to the sternum on either side 

 is a short strong bone, the coracoid; attached to it again, and 

 bending backwards over the ribs, is the scapula; and at the junction 

 of scapula and coracoid is the socket, or glenoid cavity, for the 

 wing. The clavicles, fused into a "merry-thought," run from near 

 the glenoid cavity to the front end of the keel; in strong-flying 

 birds they are stout and curved, and a continuous curve sweeps 



Fig. 485. Rough paper model of a fowl's pelvis. 



round from scapula to sternal keel. The keel is commonly explained 

 as necessary to give space enough for the attachment of the muscles 

 of flight, but this explanation is inadequate, even untrue; for one 

 thing, the great pectoral muscle springs from the edge, not from 

 the broad surface of the keel. The keel is essentially a flange, and, 

 as in a piece of T iron, adds immensely to the strength and stiffness 

 of the construction*; that it tends to give the fibres of the muscle 

 more stretch and play, and a straighter pull on the arm-bone to 

 which they run, is a secondary advantage. Strong as they are, these 

 bones are exquisitely light and thin. A great frigate-bird, with a 7-foot 

 span of wings, weighs a little over a couple of pounds, and all its 

 bones weigh about four ounces. The bones weigh less than the 

 feathers |. 



♦ T irons, if I am not mistaken, were among the many inventions of Robert 

 Stephenson, in his construction of the Menai tubular bridge a hundred years ago. 

 t Cf. R. C. Murphy, Natural History, Oct. 1939. 



