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._]

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.