210 THE STRUCTURE AND LIFE OF BIRDS cwap. 
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 Machinery—Summary. 
(1) 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 of the 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 of the hand involves 
the stretching of the 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 
