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it has been shown by M. de Lucy that the extent of surface is 
always in an inverse ratio to the weight of the winged animal ; 
the heavier the animal, the smaller its wing surface referred to a 
fixed standard — this is seen remarkably in flpng insects — the body 
is very light, but the wing surface enormous. 
3rd. The bird is now in mid-air, but the force of gravity would 
soon bring it down, notwithstanding the ijarachute-like surface of 
its expanded wings spread out to the resistance of the air — were 
not some other action brought into play to overcome the balance of 
gravity. This is the third property. Force, or the muscular power 
of depressing the expanded wing, forcibly, and rapidly, so as to 
cause the elastic column of air beneath to rebound with sufficient 
force to destroy the remaining effects of gravity, and so to equalize 
all the forces, as to leave the bird, which we have chosen as our 
illustration, ready to pursue its course at will. 
Before proceeding to consider the principal forms of progression 
and the mode in which each is accomplished, it will be as well to 
glance at the mechanism by means of which the result is brought 
about. 
The first thing which strikes us on examining the skeleton of a 
bird, is its great lightness combined with strength. The bones are 
thin and hard, not filled with marrow as in the terrestrial animals, 
but cellular in their interior and filled with air ; the broad surfaces 
for the attachment of the muscles are evidence of great muscular 
power, and the consolidation of the vertibrae of the back indicates 
firmness and solidity. The neck and extremity of the tail are free, 
but in the whole trunk there is great firmness and compactness. 
The bones forming the shoulder girdle are particidarly strong and 
well knit. The whole purpose of this beautiful arrangement is to 
give power to the wings. 
In wingless birds the keel of the sternum is altogether wanting, 
but in birds of strong flight it is very conspicuous ; the depth and 
extent of this ridge indicate the development of the pectoral muscles 
which here find attachment ; two of these muscles (tlae 1st and 3rd) 
produce the powerful downstroke of the wing, the other (the 2nd) 
is the elevator. So immensely are these miiscles developed, that 
according to Borelli they out-weigh all the other muscles of a bird’s 
body taken together, whilst in a man they are but a 70th part of 
the whole mass of muscles. 
