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. [Illustration: FIG. 23.—Left clavicle of Man, front side.] [Illustration: FIG. 24.—Left hip-bone of Wolf. _Il._, ilium. _Is._, ischium. _P._, pubis. _ac._, acetabulum.] The posterior, or _pelvic_, girdle is composed on each side of a very large, irregularly shaped bone, which is firmly attached to one or more of the coalesced vertebræ which form the sacrum and thus affords a solid support to the hind leg. Each half of the _pelvis_, or hip-bone, is made up of three elements, called respectively the _ilium_, _ischium_ and _pubis_, which are separate in the very young animal, indistinguishably fused in the adult. The three elements unite in a deep, hemispherical pit, the _acetabulum_, which receives the head of the thigh-bone, a perfect example of the “ball and socket joint.” In the inferior median line the two pubes meet and may become coalesced, in a _symphysis_, the length of which differs in various mammals. The pelvis and sacrum together form a short, wide tube, the diameter of which is normally greater in the female skeleton than in the male. The limbs are each divided into three segments, which in the anterior extremity are the arm, fore-arm and hand (or fore foot) and in the posterior extremity are the thigh, leg and foot (or hind foot), and there is a general correspondence between the structure of these segments in the fore and hind legs, however great the superficial difference. The bones of the limbs, as distinguished from those of the feet, are the _long bones_ and, except in a few very large and heavy mammals, are essentially hollow cylinders, thus affording the maximum strength for a given weight of bone; the cavity of a long bone contains the marrow and hence is called the _medullary cavity_. In the young mammal each of the long bones consists of three parts, the _shaft_, which makes up much the greater part of the length, and at each end a bony cap, the _epiphysis_. Growth takes place by the intercalation of new material between the shaft and the epiphyses; when the three parts unite, growth ceases and the animal is adult.