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Show 712 MISS B. LINDSAY ON THE AVIAN STERNUM. [JlfW > 6> posterior-lateral process is well marked; f^^Z^* Hesperornis, Dinornis, Dodo, Apteryx, Solitaire, ana (c/. fig. V.). Other noteworthy instances of a f ^ J * ™ * lateral process, probably of primitive type, are given " * « " " " » flerodiM, ^ A n u c M , Aptenodytes, and in most famibes rt tne P«S5ere., in which latter the shape of the process shows a tendency towards the filling up of the sternum. Fig. V. Archaic types, with posterior-lateral processes. 1, Sternum of Didus ineptus; 2, of Dinornis crassus, from N. Otago, N e w Zealand ; 3, of Dinornis, from Hector Bange, N e w Zealand ; 4, of Pezophaps solitaria ; 5, of Apteryx oweni ; 6, of A. mantclli. (All C.C., reduced.) r, is a fontanel imperfectly closed. Finally, it is to be remarked, if the reference to a mechanical theory of the origin of the posterior processes seems to require apology, that the very existence of the sternum may perhaps be explained by a mechanical theory. The ends of the ribs fuse, as Rathke showed, into a continuous band. Now it is not improbable that this is primarily due to the establishment of lungs. The difference between the capacity of the thorax in inspiration and expiration necessitates that the extra length of each rib shall be packed in a loop during the latter position (cf. fig. VI., 2). Now in the embryo, the thorax being in the position of rest, these loops lie close together. Thus it is easy to see how the original fusion of rib-ends may have taken place. |
