An =articulation= or =joint= is formed by the union of two or more bones
or cartilages by other tissue. Bone is the fundamental part of most
joints; in some cases a bone and a cartilage, or two cartilages, form a
joint. The uniting medium is chiefly fibrous tissue or cartilage, or a
mixture of these. Union of parts of the skeleton by muscles
(Synsarcosis), as in the attachment of the thoracic limb in the horse,
will not be considered in this section.
Joints may be classified—(_a_) anatomically, according to their mode of
development, the nature of the uniting media, and the form of the joint
surfaces; (_b_) physiologically, with regard to the amount and kind of
movement or the absence of mobility in them; (_c_) by a combination of
the foregoing considerations.
The classification of joints is still in a very unsatisfactory state,
and, unfortunately, the same term is used in various senses by
different authors. The two main subdivisions proposed by Hepburn are:
(1) Those in which the uniting medium is coextensive with the opposed
joint surfaces, and in which a direct union of these surfaces is
thereby effected. (2) Those in which the uniting medium has undergone
interruption in its structural continuity, and in which a cavity of
greater or less extent is thus formed in the interior of the joint.
This distinction is of considerable importance clinically.
Three chief subdivisions of joints are usually recognized—viz.,
=synarthroses=, =diarthroses=, and =amphiarthroses=.