[Illustration: FIG. 3.--Vertical section through an amphiarthrodial joint. b, b, the two bones; c, c, the plate of cartilage on the articular surface of each bone; Fc, the intermediate fibro-cartilage; l, l, the external ligaments.]
Movable joints, or _diarthroses_, are divided into those in which there is much and little movement. When there is little movement the term half-joint or _amphiarthrosis_ is used. The simplest kind of amphiarthrosis is that in which two bones are connected by bundles of fibrous tissue which pass at right angles from the one to the other; such a joint only differs from a suture in the fact that the intervening fibrous tissue is more plentiful and is organized into definite bundles, to which the name of _interosseous ligaments_ is given, and also that it does not synostose when growth stops. A joint of this kind is called a _syndesmosis_, though probably the distinction is a very arbitrary one, and depends upon the amount of movement which is brought about by the muscles on the two bones. As an instance of this the inferior tibio-fibular joint of mammals may be cited. In man this is an excellent example of a syndesmosis, and there is only a slight play between the two bones. In the mouse there is no movement, and the two bones form a synchondrosis between them which speedily becomes a synostosis, while in many Marsupials there is free mobility between the tibia and fibula, and a definite synovial cavity is established. The other variety of amphiarthrosis or half-joint is the _symphysis_, which differs from the syndesmosis in having both bony surfaces lined with cartilage and between the two cartilages a layer of fibro-cartilage, the centre of which often softens and forms a small synovial cavity. Examples of this are the symphysis pubis, the mesosternal joint, and the joints between the bodies of the vertebrae (fig. 3).
The _true diarthroses_ are joints in which there is either fairly free or very free movement. The opposing surfaces of the bones are lined with articular cartilage, which is the unossified remnant of the cartilaginous model in which they are formed and is called the _cartilage of encrustment_ (fig. 4, c). Between the two cartilages is the _joint cavity_, while surrounding the joint is the _capsule_ (fig. 4, l), which is formed chiefly by the superficial layers of the original periosteum or perichondrium, but it may be strengthened externally by surrounding fibrous structures, such as the tendons of muscles, which become modified and acquire fresh attachments for the purpose. It may be said generally that the greater the intermittent strain on any part of the capsule the more it responds by increasing in thickness. Lining the interior of the capsule, and all other parts of the joint cavity except where the articular cartilage is present, is the _synovial membrane_ (fig. 4, dotted line); this is a layer of endothelial cells which secrete the synovial fluid to lubricate the interior of the joint by means of a small percentage of mucin, albumin and fatty matter which it contains.