Fig. 23.—Left clavicle of Man, front side.
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.
Fig. 25.—Left humerus of Wolf, from the front and outer sides, the latter somewhat oblique. h., head. int.t., internal tuberosity. ext.t., external tuberosity. bc., bicipital groove. dt., deltoid ridge. sh., shaft. s., supinator ridge. int. epi., internal epicondyle. s.f., anconeal foramen. tr., trochlea. tr′., trochlea, posterior side. ext. epi., external epicondyle. a.f., anconeal fossa.
Fig. 26.—Left humerus of Horse, front side. i.t., internal tuberosity. ex.t., external tuberosity. bc., outer part of bicipital groove. dt., deltoid ridge. s., supinator ridge. tr., trochlea.
