Page:Popular Science Monthly Volume 60.djvu/326

318 But it was in the fore limbs and their attachments that the remarkable adaptation for flight of these animals is found. The shoulder-blade and coracoid, functionally replacing the collar-bone of many animals, were fused together into a stout curved bone, articulating in front with the broad breast-bone and behind with the spines of several fused back-bones or vertebrae. These two shoulder bones thus formed a complete, stout ring, firmly attached in front and behind. Such a mode of articulation of these bones is found in no other animal. To add further to the strength and rigidity of the thorax, the strong upper ribs were not movably articulated, but coossified with the vertebrae. Altogether, the large and broad breast-bone, the complete shoulder-ring, the rigidly fixed upper ribs and the immovable vertebrae furnished a support for the attachment of the enormous wing and its necessary muscles that has no parallel for strength among other animals.

Among the higher animals there are three types of structure whereby the fore limbs are modified into organs of flight. In birds, the arm and fore-arm bones are elongated and strengthened; the bones of the wrist are fused together or wanting, while the few bones of the fingers are welded together and made to hinge sideways. In the bat, among mammals, all the bones, save those of the wrist, are elongated and slender. The very slender fingers are spread out fan-like laterally to support the membrane which replaces the feathers of the bird.

In the pterodactyls, the volant surface, as in the bats, was presented by a thin membrane, which extended from the fore limb to the sides of the body. The bones of the arm were strong, with strong projections for the attachment of muscles; those of the wrist were few in number and closely united. The rudimentary first finger, or thumb, consisting of a single slender bone, was turned backward toward the shoulder for the support of the membrane in front of the elbow. The second, third and fourth fingers were entire, but very small and slender, and were provided at their extremities with very sharp and strongly curved claws. While these fingers were flexible and prehensile, their small size and weakness of attachment (they did not articulate with the wrist) suggest that their only use was for clinging. In strange contrast with these small fingers, the fifth digit, that is the one corresponding to the little finger of the human hand, was enormously elongated and strong, including by far the largest bones of the entire skeleton. Between