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.
Fig. 20.—Left scapula of Wolf. gl., glenoid cavity. c., coracoid. ac., acromion. sp. spine.
Fig. 21.—Left scapula of Horse. This figure is much more reduced than [Fig. 20].
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.
The collar-bone, or clavicle, is a complexly curved bar, which, when present and fully developed, extends from the forward end of the sternum to the acromion, the projecting lower end of the scapular spine, supporting and strengthening the shoulder-joint. In many mammalian orders, notably all existing hoofed animals, the clavicle has become superfluous and is lost more or less completely; it may be said, in general, that the clavicle is developed in proportion to the freedom of motion of the shoulder-joint and to the power of rotation of the hand upon the arm. In arboreal animals, such as monkeys, in which the hand rotates freely and the arm moves in any direction on the shoulder, the clavicle is large and fully developed, as it also is in Man. Many burrowing mammals (e.g. the moles) have very stout clavicles.
