The bones of the skeleton are joined together in various ways to form joints. The manner in which they are united will depend upon the purpose they are intended to perform, hence joints are divided into three classes according to their respective range of movement. 1. Diarthrodial joints, which enjoy the greatest freedom of action. 2. Amphiarthrodial joints, whose powers of movement are much more restricted. 3. Syn-arihrodial, or those which are fixed and immovable.

Diarthrodial or Free-moving Joints are composed of two or more bones whose articular surfaces are covered with a thin layer of cartilage or gristle, and so formed as to permit one to play freely upon the other. They are all enclosed in a sac lined by a delicate membrane for the secretion of synovia or joint-oil, and for the most part the bones are held together by connecting ligaments.