FLIGHT OF BIRDS. 
321 
Power of flight is seconded by great acuteness of vision, 
Milch enables them, as tliey travel at a swift rate, to 
tiew objects below them, to discover their food with 
facility, 'and thus to finish their journey. Thus, it has 
‘he wings, bear all their force upon the sternum, through the inter- 
v tmtion of which they make the trunk perform a kind of leap ; and 
h l e wings, the immediate lowering of which the atmospheric fluid 
resists, as we have just said, are nevertheless lowered by this 
^WhilealTThese things are performing with an extreme velocity, 
several muscles of the arm, among others the extensors of the lore- 
»r m , try to extend the wing ; hut as the resistance of the air upon 
tile extremities of the wings is very great, and as that fluid 
“I'poses all rapid motion on their part, these muscles then 
direct their force towards the trunk. Taking, therefore, their 
t*ed point in the bones of the fore-arm, on the outer side of the 
*iug, P and acting by their upper extremity, they extend the arm 
kpon the fore-arm ; and as this action, and that of the great pec- 
toral muscles, take place at the same moment and m concert at both 
sides of the trunk, the latter is pushed upwards in a middle direc- 
tion • i 
Thus, the combination of these various efforts impress upon the 
hunk a force of projection which carries it upwards and forwaids 
along With the wings; and this projection U evident relations 
to the leap of other animals. Then the great pectoral muscles 
htlax, and the wings rise immediately, partly by the reaction of the 
ah upon their lowir surface, and by the descent of the trunk, and 
Partly by the action of the middle pectoral muscles and their con- 
Sdiers, whose contraction is in a manner permanent, during Uigur. 
The bird, after leaping forwards, no longer weighing upon the 
4lt during a moment, that fluid then, by -ts reaction, repels it, and 
Sis to raise it higher than the leap alone could have made it do. 
it then prevents it from falling so low as the noint of departure. 
. The Lent of the trunk is, without doubt, favoured by the 
Eternal air, which introduces itself into all parts of the animal, and 
Hich it has the faculty of retaining. This air, which is perhaps 
1 light gas, being dilated and rarefied by great heat, not only is its 
"Pacific gravity thereby greatly diminished, but it must contribute 
to diminish that of the bird by filling up all its vacuities during 
• The bird which descends with precipitation, if it be ufraidof 
“burin., itself in approaching the ground, opens its wings and tail, 
performs aevenS small leap*, which, diminishing the rapidity 
uf the descent, nermit it to alight gently. ,, , 
, h is by means of the tail that certain birds are enabled to 
^end from great heights with precipitation: by bringing the 
VOL. IV. X U 
