210 THE STRUCTURE AND LIFE OF BIRDS CHAP. 



less than four ; the second digit (the first finger as we 

 call it in our own hands) has three, and the third digit 

 only one. But it must be remembered that these two 

 digits are fixed together so that one cannot move 

 without the other. Counting long and short muscles 

 together we have thirteen to move the " thumb " and 

 fingers. Of these the " thumb " has five, one long and 

 four short. 



Active and Passive Machineiy — Summary. 



(i) All the muscles of any size at all arise not 

 further from the body than the far end of the humerus 

 or the near end ofthe forearm. All the really large 

 muscles arise from the body. The spreading of 

 the wing is much simplified by the fact that the radius, 

 when the elbow-joint is straightened, slides back and 

 extends the hand. The extension ofthe hand involves 

 the stretching ofthe elastic ligament, and, consequently, 

 the spreading of the great feathers. The straightening 

 of the arm also spreads the anterior and axillary 

 membranes, thereby greatly increasing the wing area. 



(2) The fingers must have a great deal of freedom 

 of movement, since they have so many muscles 

 attaching to them. Similar evidence seems to show 

 that the " thumb " cannot be quite rudimentary. 

 Later on I shall show that it is sometimes used. 



(3) Muscles, which, if worked separately, have con- 

 trary effects, may work together with good results. 

 Thus if the muscle which bends the wrist be contracted 

 at the same time as the one which puts the hand in a 



