On Motion in the lumbar division of the spine of birds, by George Oakley Fleming

                                them; and by this arrangement nature seems to have 
saved weight and space, as well as to have obtained 
the object for which they were more especially intended, 
that of restraining backward motion, as in the sea-gull, 
(See Plate 2, <s>Fig:<\s> letters a). And as Dr Roget has observed, 
of the vane of the feather in the wings of birds “they 
derive this power of resistance from their flattened 
shape, which allows them to bend less easily in the 
direction of their flattened surfaces, than in any other; 
in the same way, that a slip of card cannot easily 
be bent by a force acting in its own plane, though 
it easily yields to one at right angles to it. Now 
it is exactly in the direction in which they do not 
bend, that the filaments of the feather have to encounter 
the resistance and impulse of the air; it is here, that 
strength has been wanted, and it is here, that strength 
has been bestowed.” Bridgewater Treatise, Vol. 1.:p. 569. 
The motion of the spine forwards is prevented by 
strong flat ligaments, in color very like the ligamentum 
nuchae, which is situated between each spinous process, 
and attached to the thin edge of each, the whole of its 
length. The long flat bones, which I have just des-
cribed, exert their influence too, in restraining the 
forward motion of the spine; and in some birds, as 
the