| OCR Text |
Show 710 MISS B. LINDSAY ON THE AVIAN STERNUM. [Juiiel6, separate nuclei for the trochanters of the femur, or the epiphyses long bones. It will be seen that the above conclusions lead us to a much more complete and detailed idea of the nature of the parts of the sternum than at first sight it appeared possible to attain. If the attempt to form a complete theory of the origin of the sternum appears a rash undertaking, in the face of the opinion expressed by Prof. Parker, that our anatomical knowledge of the parts is as yet insufficient tor teleclogical interpretation, it must be remembered that the study of five types has afforded a variety of new and interesting facts, and we may at least expect that wider investigation will shortly afford a true Fig. IV. Left figure, theoretical diagram of the Ostrich sternum; right figure, of Rhea; middle figure, of Chick. The costal sternum is left white; ac, pc, indicate its anterior and posterior parts, produced by ribs atrophied in the adult; the metasternum is shaded vertically, also the anterior-lateral process. Additions of later phylogenetic date are shaded horizontally. In the Ostrich and Bhea, B, indicates the limit of the ossification in the adult; o, the dotted spaces, are the centres of ossification in the young; /, is a fontanel, scarcely closed by thin bone in some specimens. solution of the problems under consideration, even if in the present instance the effort to attain it is unsuccessful. The theoretical construction of the sternum thus adopted will be best explained by reference to the above diagrammatic representation of the sternum in Fig. IV., and to the following tabular classification of the parts of the Avian sternum. That the boundaries of ossification have not alwavs a phylogenetic significance is clearly shown in the Ostrich. Comparison of the embryos (Plate X L I L ) shows that the primary costal halves of the sternum become united by cartilage developed in the median line, yet the line of ossification is as indicated in the diagram. |
