FIG. 56. THREE KINDS OF WINGS. 



A. The wing and associated parts of an extinct flying reptile, a Ptero- 

 dactyl or Pterosaur. An extension of skin, the mark of which is seen on 

 some of the fossils, was held out by the arm and mainly by the greatly 

 elongated outermost finger. The first digit is rudimentary or absent, so 

 the one which is exaggerated is " the little finger," No. V. Thence the 

 skin or patagium was continued to the hind leg and even to the tail. 



B. The wing and associated parts of a bat. The extension of skin or 

 patagium begins at the side of the neck, passes along the upper side of the 

 arm to the wrist, practically skips the thumb, is continued between the 

 four fingers and thence doAyn the side of the body to the hind leg, and 

 between the legs to the tail, if there is one. 



C. The wing of a bird. In front there is a small patagium from the 

 upper arm to the wrist, but the surface for striking the air, which was 

 gained in the other forms by the skin, is here due to the feathers. A small 

 tuft, the ala spuria, is attached to the thumb. The longest feathers, the 

 primaries, are fastened to a fused bone called the carpo-metacarpus (due 

 to a coalescence of three wrist-bones or carpals and three palm-bones or 

 metacarpals). The other feath rs shown, the secondaries, are attached 

 to the ulna, the outer surface of which shows a row of tuberosities where 

 the muscles which move the individual feathers a little are inserted. 

 Besides the feathers shown, there are smaller wing-coverts above and 

 below. 



These three flying structures are analogous. The bird's wing is homo- 

 logous with the anterior part the arm part of the flying apparatus of 

 the two others. Some believe that birds had originally a posterior pata- 

 gium from the upper part of the leg to the tail, which once was long. 



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