Fig. 17.—Caudal vertebræ of Wolf, from anterior and middle parts of the tail. Letters as in [Fig. 15].
(5) The caudal vertebræ vary greatly, in accordance with the length and thickness of the tail. In an animal with well-developed tail several of the anterior caudals have the parts and processes of a typical vertebra, centrum, neural arch and spine, zygapophyses and transverse processes. Posteriorly, these gradually diminish, until only the centrum is left, with low knobs or ridges, which are the remnants of the various processes. A varying number of long, cylindrical centra, diminishing backward in length and diameter, complete the caudal region and the vertebral column. In some mammals, chevron bones are attached to the under side of the anterior and middle caudals; these are forked, Y-shaped bones, which form a canal for the transmission of the great blood-vessels of the tail.
Fig. 18.—Ribs of Wolf from anterior and middle parts of the thorax. cp., head, t., tubercle.
The ribs, which are movably attached to the backbone, together with the dorsal vertebræ and breast-bone, compose the thorax, or chest. The articulation with the vertebræ is by means of a rounded head; in most cases the head has two distinct facets, the pit being formed half on the hinder border of one dorsal vertebra and half on the front border of the next succeeding one, but posteriorly the pit is often shifted, so as to be on a single vertebra. A second articulation is by means of the tubercle, a smooth projecting facet on the convexity of the rib’s curvature and near the head; the tubercle articulates with the transverse process of its vertebra. The ribs, in general, are curved bars of bone, which in small mammals generally and in the clawed orders are slender and rod-like, while in the hoofed mammals they are broader, thinner and more plate-like, especially the anterior ones. The number of pairs of ribs is most commonly 13, but ranges among existing mammals from 9 in certain whales to 24 in the Two-toed Sloth (Cholœpus didactylus). The complex curvature of the ribs, outward and backward, is such that, when they are drawn forward (in Man upward) by muscular action, the cavity of the thorax is enlarged and air is drawn into the lungs, and when they are allowed to fall back, the cavity is diminished and the air expelled.
Below, a varying number of the ribs are connected by the cartilages in which they terminate with the breast-bone (sternum); sometimes these cartilages are ossified and then form the sternal ribs, but there is always a flexible joint between the latter and the true ribs. In certain edentates, notably the anteaters and the extinct †ground-sloths, these sternal ribs, at their lower ends, are provided with head and tubercle, for articulation with the sternum.
The sternum, or breast-bone, is made up of a number of distinct segments, usually broad and flat, but often cylindrical, which may unite, but far more commonly remain separate throughout life. The number, size and form of these segments often give useful characters in classification. The first segment, or manubrium, has quite a different shape from the succeeding ones and is considerably longer.
Fig. 19.—Sternum and rib-cartilages of Wolf, lower side. P.S., manubrium. X.S., xiphisternum.