are persistently free for a very long period, and in some cases never become ankylosed with their vertebrae. But it should be noted that in this group there is no approximation to the state of affairs which exists in many lower Vertebrates, where there is a gradual transition between the ribs of the cervical and those of the dorsal region of the vertebral column; for that of the seventh ribs in Monotremes is smaller than those which precede it.
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Fig. 18.—Sternum and sternal ribs of the Common Mole (Talpa europaea), with the clavicles (cl) and humeri (H); M, manubrium sterni. Nat. size. (From Flower's Osteology.) |
Fig. 19.—Sternum of the Pig (Sus scrofa). × ¼. ms, Mesosternum; ps, presternum; xs, xiphisternum. (From Flower's Osteology.) |
The Sternum.—All the Mammalia so far as is known possess a sternum. This is the bone, or series of bones (sternebrae), which lies upon the ventral surface of the chest, and to which the ribs are attached below. The development of the sternum has been shown to take place from the fusion of the ribs below into two lateral bands, one on each side; the approximation of these bands forms the single and unpaired sternum of most mammals. Very considerable traces, however, of the paired state of the sternal bones often exist; thus in the Sperm Whale the first piece of the sternum is divided into two by a longitudinal division, and the second piece is longitudinally grooved. The development of the sternum out of the fused ends of ribs is shown in a more complete condition in some species of Manis than in many other mammals. Thus in M. tricuspis the last ribs of those which are attached to the sternum are completely fused together into a single piece on each side.[1] As a general rule the last ribs which come into relation with the sternum do so only in an imperfect way, being simply firmly attached at their sides to, but not fused with, the last ribs which are definitely articulated with the sternum. Contrary to what is found in lower Ver-
- ↑ Ehler's Zool. Miscellen, i. 1894.
