Most of the movements which we call bending the spine are really movements of other joints which connect the body or head with it.

When we bend our necks, for instance, we hardly bend the backbone at all, as most of the movement is made in the joint at the top of it, between it and the skull.

Similarly, when we bend our backs, we really bend our backbones very little; for most of the movement comes at the hip joints, between the thighs and the hip bones.
Each of the limbs has a single, long, rounded bone in the upper part, known in the arm as the _humerus_, and two bones in the lower part.
These last are known as the _radius_ and _ulna_ (the "funny bone") in the forearm, and the _tibia_ and _fibula_ in the leg.

The shoulder-joint is made by the rounded head of the humerus fitting into the shallow cup of the _scapula_, or shoulder-blade.

It is shallower than the hip joint to allow it freer movement; but this makes it weaker and much more easily dislocated, or put out of joint,--the most so, in fact, of any joint in the body.
[Illustration: A BALL-AND-SOCKET JOINT Hip joint.] [Illustration: A HINGE JOINT Knee joint, with the knee cap removed] The hip joints are deep, strong, cup-shaped sockets upon each side of the hip bones, or _pelvis_, into which fit the heads of the _femurs_ or thigh bones.