Thoracic ribs.
The Crocodile has ten pairs of thoracic ribs, all except the last one or two of which consist of three parts,—a vertebral rib, an intermediate rib and a sternal rib.
Of the vertebral ribs the third may be taken as a type, it consists of a curved bony rod which articulates proximally with the transverse process of the vertebra by two facets. The terminal one of these, the capitulum or head, articulates with a notch on the side of the transverse process; the other, the tuberculum, which lies on the dorsal surface a short distance behind the head, articulates with the end of the transverse process. From near the distal end an imperfectly ossified uncinate process (see p. 190) projects backwards.
The intermediate ribs are short and imperfectly ossified; they are united with the sternal ribs (fig. 46, 3), which are large, flattened, likewise imperfectly ossified structures, and articulate at their distal ends with a pair of long divergent xiphisternal horns (fig. 46, 5), which arise from the posterior end of the sternum proper. The last pair of sternal ribs are attached to the preceding pair, not to the xiphisternal horns.
The first and second vertebral ribs differ from the others in the fact that the tuberculum forms a fairly long outstanding process.
Cervical ribs.
Movable ribs are attached to all the cervical as well as to the thoracic vertebrae. Those borne by the atlas and axis are long, narrow structures attached by a fairly broad base, and tapering gradually. The ribs borne by the third to seventh cervical vertebrae are shaped like a T with a double base, one limb of which, corresponding to the tuberculum (fig. 41, 7), articulates with a short transverse process arising from the neural arch, while the other, corresponding to the capitulum, articulates with a surface on the centrum. The ribs attached to the eighth and ninth cervical vertebrae are intermediate in character between the T-shaped ribs and the ordinary thoracic ribs. The anterior limb of the T is shortened, the posterior one is drawn out, forming the shaft of the rib. The distal portion of the rib of the ninth cervical vertebra is unossified.
The Sacral ribs have been described in connection with the sacral vertebrae.
The Sternum.
The sternum of Crocodiles is a very simple structure, consisting of a plate of cartilage (fig. 46, 2) lying immediately dorsal to the interclavicle, and drawn out posteriorly into a pair of long xiphisternal horns (fig. 46, 5).