. The structure and life of birds . ceeds very far withthe digits of birds feet, or with those of the fore orhind feet of mammals, they are lost, as far as can be,symmetrically, not in lopsided fashion. And since itis the ulnar side of the wing on which mainly thestrain falls in flight, it is not likely that all the weak-ening would go on on this side and all the strength-ening on the other. Moreover, in the embryo Hoatzinthere has been found beyond the so-called thumb,besides vaguely suggestive cartilage, a bone, small yetsolid and well defined, that may be a trace of the truethumb that has d
. The structure and life of birds . ceeds very far withthe digits of birds feet, or with those of the fore orhind feet of mammals, they are lost, as far as can be,symmetrically, not in lopsided fashion. And since itis the ulnar side of the wing on which mainly thestrain falls in flight, it is not likely that all the weak-ening would go on on this side and all the strength-ening on the other. Moreover, in the embryo Hoatzinthere has been found beyond the so-called thumb,besides vaguely suggestive cartilage, a bone, small yetsolid and well defined, that may be a trace of the truethumb that has In any case, the birds 1 See Leighton, Tuffs College Studies TIL, on The Develop-ment of the Wing of-Sterna Wilsonii, and W. K. Parker,Trans. Zool. Soc, Part 2, April, 1891, on The Morphology ofOpisthocomus Cristatus 44 THE STRUCTURE AND LIFE OF BIRDS chap. wing is very different from the pterodactyls. To thesupport of the latter only one finger, often called theulnar finger since it articulates with the postaxial side. Fig. 17.— (1) Pterodactylus Spectabilis, from lithographic-stone, Bavaria. of the arm, contributes, and this one may be either or No. 5. The settlement of the question dependsupon the nature of the small bone which can be seen IV CONNECTING LINKS 45 M - s-1 O Q. tII o 2bo i» 1 Q. 46 THE STRUCTURE AND LIFE OF BIRDS chap. projecting from the wrist, and which may be a remnantof the first metacarpal or only a sesamoid bone [see (1)]. The wing is a great sheet of membrane sup-ported by this ulnar finger, which was of enormous,length, and also by the leg and tail. Thus, whereastwo fingers united help in the formation of the birdswing, only one forms part of the pterodactyls, for theother three are little clawed appendages of no use inflight. Whether these fingers in the bird be Nos. 3 and4, or 2 and 3 makes little difference. There are two,not one only, and there is no sign that the smaller oneis likely to disappear, and the larger one bein
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