THE MACHINERY OF FLIGHT 83 
There is another ligament which fastens to the 
under-side of the quills of the secondaries nearer to 
their base. Moreover, the secondaries grow back- 
ward, and plant themselves each in a little pocket 
of very tough fibrous tissue. These pockets are 
firmly rooted in the hinder of the two bones of the 
fore-arm (the ulna) and leave, each of them, their 
mark on it (see fig. 23). Planted thus, the second- 
aries, though very firmly gripped, have freedom to - 
rotate. The primaries, pointing outward as they do, 
run along above the hand and finger bones and are 
held with a still stronger grip. 
It is marvellous machinery which spreads the 
wings so rapidly, and which, having all its heaviest 
parts massed upon the body or very near to it, 
leaves them splendidly light, especially near their 
extremities. But we have not yet come to the end 
of the contrivances. During the down-stroke the , 
secondaries press each against the one above it and 
so prevent the passage of air, whereas during the 
upstroke they let it pass. They are held fast by a 
number of stays; there are the little pockets just 
described, the ligaments and a sheet of fibrous 
tissue extending from their bases to the great liga- 
ment. There are also little tendons that connect 
the secondaries with a muscle which, arising from 
the further end of the humerus, attaches its other 
extremity to one of the wrist-bones (carpals) and to 
one of the metacarpals, or hand-bones, beyond (see 
fig. 24). Working unopposed it bends the wrist. 
When the wing is straightened it helps by its opposi- 
tion to hold the wrist-joint tight, thus strengthening 
a point where the strain of flight is much felt. When 
G2 
