ADAPTATIONS OF THE SKELETON 55 



mobility of the parts of the wing, to make the wing act as 

 one instrument when it strikes, and to tuck it up when at 

 rest in a space-economising fashion — especially useful when 

 the bird is swimming with its feet. 



The radius and ulna are quite free from one another, 

 but they do not move on one another except that in the 

 folding up of the wing the radius moves forward a little 

 on the ulna and thus helps to bring the hand into its resting 

 position. 



The embryo bird shows four or five wrist-bones or 

 carpals ; but these are reduced to two free bones — the 

 radiale and the ulnare. This is in relation to the reduced 

 mobility of the whole limb. What becomes of the other 

 carpals ? Three of them fuse on to the three fused meta- 

 carpals, forming the compound bone known as the carpo- 

 metacarpus. This may be described as consisting of half 

 of the wrist and the whole of the palm. It affords the firm 

 foundation for the primaries — the longest feathers of flight. 

 At its upper end it bears the single joint or phalanx of the 

 thumb ; at its lower end it bears the second digit with two 

 joints, and the third digit with one. 



There is some uncertainty in regard to the digits that 

 are now represented in the bird's wing. Some authorities 

 say that the strongest one is the third, and that those on the 

 two sides of it are the second and fourth. In other words, 

 the first and fifth have disappeared. We adhere to the 

 interpretation of the bird's three digits as I, II, and III. 

 But the important fact is that the bird's hand is relatively 

 rudimentary. It has been sacrificed to making a wing — 

 that is to say, a unified basis for the pinions. 



(6) In the oldest known bird, Archasopteryx, there was 

 a long tail like a lizard's, with a very peculiar arrangement 

 of feathers in a row on each side. Such a tail doubtless had 

 some use as a steering organ. In modern birds the tail in 

 the strict sense is always short, and in Flying Birds it ends 

 in a fusion, the ploughshare bone or pygostyle. If there 

 seems to be a long tail in a living bird, as in the pheasant, 

 it is always a feather tail. The ploughshare bone may be 



