THE HIP BONES, called by anatomists the innominata, or nameless bones, form an irregular basin styled the pelvis (pelvis, a basin). In the upper part, is the foot of the spinal column—a wedge-shaped bone termed the sacrum [Footnote: So called because it was anciently offered in sacrifice.] (sacred), firmly planted here between the widespreading and solid bones of the pelvis, like the keystone to an arch, and giving a steady support to the heavy burden above.
3. THE LIMBS.
TWO SETS OF LIMBS branch from the trunk, viz.: the upper, and the lower. They closely resemble each other. The arm corresponds to the thigh; the forearm, to the leg; the wrist, to the ankle; the fingers, to the toes. The fingers and the toes are so much alike that they receive the same name, digits, while the several bones of both have also the common appellation, phalanges. The differences which exist grow out of their varying uses. The foot is characterized by strength; the hand, by mobility.
FIG. 10.
[Illustration: The Shoulder Joint. a, the clavicle; b, the scapula.]
1. THE UPPER LIMBS.—THE SHOULDER.—The bones of the shoulder are the collar bone (clavicle), and the shoulder blade (scapula). The clavicle (clavis, a key) is a long, slender bone, shaped like the Italic f. It is fastened at one end to the breastbone and the first rib, and, at the other, to the shoulder blade. (See Fig. 1.) It thus holds the shoulder joint out from the chest, and gives the arm greater play. If it be removed or broken, the head of the arm bone will fall, and the motions of the arm be greatly restricted. [Footnote: Animals which use the forelegs only for support (as the horse, ox, etc.), do not possess this bone. "It is found in those that dig, fly, climb and seize.">[
THE SHOULDER BLADE is a thin, flat, triangular bone, fitted to the top and back of the chest, and designed to give a foundation for the muscles of the shoulder.
THE SHOULDER JOINT.—The arm bone, or humerus, articulates with the shoulder blade by a ball-and-socket joint. This consists of a cup-like cavity in the latter bone, and a rounded head in the former, to fit it,— thus affording a free rotary motion. The shallowness of the socket accounts for the frequent dislocation of this joint, but a deeper one would diminish the easy swing of the arm.
FIG. 11.
[Illustration: Bones of the right Forearm. H, the humerus;
R, the radius; and U, the ulna.]