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. [Illustration: FIG. 19.—Sternum and rib-cartilages of Wolf, lower side. _P.S._, manubrium. _X.S._, xiphisternum.] II. The appendicular skeleton consists of the limb-girdles and the bones of the limbs and feet. The limb-girdles are the means of attaching the movable limbs to the body, so as to combine the necessary mobility with strength. The anterior, or _shoulder-girdle_, has no direct articulation with the vertebral column, but is held in place by muscles; it is made up of the shoulder-blade and collar-bone, though very many mammals have lost the latter. [Illustration: FIG. 20.—Left scapula of Wolf. _gl._, glenoid cavity. _c._, coracoid. _ac._, acromion. _sp._ spine.] [Illustration: FIG. 21.—Left scapula of Horse. This figure is much more reduced than Fig. 20.] [Illustration: FIG. 22.—Left scapula of Man in position of walking on all fours. Letters as in Fig. 20.] The shoulder-blade, or _scapula_, is a broad, thin, plate-like bone, which contracts below to a much narrower neck, ending in a concave articular surface, the _glenoid cavity_, for the head of the upper arm-bone, the two together making the shoulder-joint. On the outer side the blade is divided into two parts by a prominent ridge, the _spine_, which typically ends below in a more or less conspicuous projection, the _acromion_, which may, however, be absent, its prominence being, generally speaking, correlated with the presence of the collar bone. A hook-like process, the _coracoid_, rises from the antero-internal side of the glenoid cavity and varies greatly in size in the different groups of mammals; though it usually appears to be merely a process of the scapula, with which it is indistinguishably fused, yet its development shows it to be a separate element and in the lowest mammals (Prototheria), as in the reptiles and lower vertebrates generally, it is a large and important part of the shoulder-girdle and articulates with the sternum.