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 



