FLIGHT OF BIRDS, 
321 
power of flight is seconded by great acuteness of vision, 
which enables them, as they travel at a swift rate, to 
view objects below them, to discover their food with 
facility, and thus to finish their journey. Thus, it has 
the wings, bear all their force upon the sternum, through the inter- 
vention of which they make the trunk perform a kind of leap ; and 
the wings, the immediate lowering of which the atmospheric fluid 
resists, as we have just said, are nevertheless lowered by this 
indirect means. 
While all these things are performing with an extreme velocity, 
several muscles of the arm, among others the extensors of the fore- 
arm, try to extend the wing ; but as the resistance of the air upon 
the extremities of the wings is very great, and as that fluid 
opposes all rapid motion on their part, these muscles then 
direct their force towards the trunk. Taking, therefore, their 
fixed point in the bones of the fore-arm, on the outer side of the 
wing, and acting by their upper extremity, they extend the arm 
upon the fore-arm ; and as this action, and that of the great pec- 
toral muscles, take place at the same moment and in concert at both 
sides of the trunk, the latter is pushed upwards in a middle direc- 
tion. 
Thus, the combination of these various efforts impress upon the 
trunk a force of projection which carries it upwards and forwards 
along with the wings ; and this projection has evident relations 
to the leap of other animals. Then the great pectoral muscles 
relax, and the wings rise immediately, partly by the reaction of the 
air upon their lower surface, and by the descent of the trunk, and 
partly by the action of the middle pectoral muscles and their con- 
geners, whose contraction is in a manner permanent during flight. 
The bird, after leaping forwards, no longer weighing upon the 
air during a moment, that fluid then, by its reaction, repels it, and 
tends to raise it higher than the leap alone could have made it do. 
It then prevents it from falling so low as the point of departure. 
The ascent of the trunk is, without doubt, favoured by the 
internal air, which introduces itself into all parts of the animal, and 
which it has 1 the faculty of retaining. • This air, which is perhaps 
a light gas, being dilated and rarefied by great heat, not only is its 
specific gravity thereby greatly diminished, but it must contribute 
to diminish that of the bird by filling up all its vacuities during 
flight. 
The bird which descends with precipitation, if it be afraid of 
injuring itself in approaching the ground, opens its wings and tail, 
and performs several small leaps, which, diminishing the rapidity 
of the descent, permit it to alight gently. 
It is by means of the tail that certain birds are enabled to 
descend from great heights with precipitation : by bringing the 
VOL. IV. X II 
