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._] THE ELBOW.--At the elbow, the humerus articulates with the _ulna_--a slender bone on the inner side of the forearm--by a hinge joint which admits of motion in only two directions, _i. e._, backward and forward. The ulna is small at its lower end; the _radius_, or large bone of the forearm, on the contrary, is small at its upper end, while it is large at its lower end, where it forms the wrist joint. At the elbow, the head of the radius is convex and fits into a shallow cavity in the ulna, while at the wrist the ulna plays in a similar socket in the radius. Thus the radius may roll over and even cross the ulna. THE WRIST, or _carpus_, consists of two rows of very irregular bones, one of which articulates with the forearm; the other, with the hand. They are placed side to side, and so firmly fastened as to admit of only a gliding motion. This gives little play, but great strength, elasticity, and power of resisting shocks. THE HAND.--The _metacarpal_ (_meta_, beyond; _karpos_, wrist), or bones of the palm, support each a thumb or a finger. Each finger has three bones, while the thumb has only two. The first bone of the thumb, standing apart from the rest, enjoys a special freedom of motion, and adds greatly to the usefulness of the hand. FIG. 12. [Illustration: _Bones of the Hand and the Wrist._] The first bone (Figs. 11, 12) of each finger is so attached to the corresponding metacarpal bone as to move in several directions upon it, but the other phalanges form hinge joints. The fingers are named in order: the thumb, the index, the middle, the ring, and the little finger. Their different lengths cause them to fit the hollow of the hand when it is closed, and probably enable us more easily to grasp objects of varying size. If the hand clasps a ball, the tips of the fingers will be in a straight line.