1014 ON FORM AND MECHANICAL EFFICIENCY [ch. 



to speak) take hold of and support, and which is now, mechanically 

 speaking, the axis of the body. The bird has two points of suspension, 

 which it uses alternately: the one through the two acetabula, the 

 other through the glenoid cavities and the outstretched wings. 

 Glenoid cavity and acetabulum are but a little way apart, and the 

 bird swings its weight over from one to the other easily and smoothly. 

 At first sight it seems a curious feature of the bird's skeleton that 

 breast-bone, shoulder-girdle, wings and all are but very slightly 

 attached to the rest of the body, and to what we look on, usually, 

 as its main axis of support ; the only skeletal attachment is by the 

 framework of the ribs, and these are sUght and slender. The fact 

 is that the two skeletal axes, the backbone and the breast-bone, have 

 their separate and independent roles, and each is but loosely connected 

 with the other. 



Fig. 488. Diagram of a continuous girder. 



The curvature of the bird's neck is very beautiful: one curve 

 leads on to another; and indeed the bird's whole axial skeleton, 

 from head to tail, is one even and continuous curve. Where a 

 bridge crosses the gap between two piers, it sags as the load passes 

 over; where successive girders cross successive gaps, each sags in 

 its turn under the travelHng load. But suppose one continuous 

 girder to cross two gaps; it bends in a more complicated way, and 

 one half tends to bend up while the other is sagging down. We 

 cannot analyse the whole field of force to which the bird is subject, 

 but we reahse that it is a continuous field, in which what the engineer 

 calls a "continuous girder" has its great part to play. The 

 continuous girder is apt to sag and bend and sway in an erratic 

 fashion unless its ends be firm and secure, and the bird's head must, 

 of necessity, be under some analogous control; the semicircular 

 canals are the potent factors in equilibrmm, and the bird "keeps 



