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.

Fig. 4.—Vertical section through a diarthrodial joint. b, b, the two bones; c, c, the plate of cartilage on the articular surface of each bone; l, l, the investing ligament, the dotted line within which represents the synovial membrane. The letter s is placed in the cavity of the joint.	Fig. 5.—Vertical section through a diarthrodial joint, in which the cavity is subdivided into two by an interposed fibro-cartilage or meniscus, Fc. The other letters as in fig. 4.

A compound diarthrodial joint is one in which the joint cavity is divided partly or wholly into two by a meniscus or interarticular fibro-cartilage (fig. 5, Fc).

The shape of the joint cavity varies greatly, and the different divisions of movable joints depend upon it. It is often assumed that the structure of a joint determines its movement, but there is something to be said for the view that the movements to which a joint is subject determine its shape. As an example of this it has been found that the mobility of the metacarpo-phalangeal joint of the thumb in a large number of working men is less than it is in a large number of women who use needles and thread, or in a large number of medical students who use pens and scalpels, and that the slightly movable thumb has quite a differently shaped articular surface from the freely movable one (see J. Anat. and Phys. xxix. 446). R. Fick, too, has demonstrated that the concavity or convexity of the joint surface depends on the position of the chief muscles which move the joint, and has enunciated the law that when the chief muscle or muscles are attached close to the articular end of the skeletal element that end becomes concave, while, when they are attached far off or are not attached at all, as in the case of the phalanges, the articular end is convex. His mechanical explanation is ingenious and to the present writer convincing (see Handbuch der Gelenke, by R. Fick, Jena, 1904). Bernays, however, pointed out that the articular ends were moulded before the muscular tissue was differentiated (Morph. Jahrb. iv. 403), but to this Fick replies by pointing out that muscular movements begin before the muscle fibres are formed, and may be seen in the chick as early as the second day of incubation.

The freely movable joints (true diarthrosis) are classified as follows:—

(1) Gliding joints (Arthrodia), in which the articular surfaces are flat, as in the carpal and tarsal bones.

(2) Hinge joints (Ginglymus), such as the elbow and interphalangeal joints.

(3) Condyloid joints (Condylarthrosis), allowing flexion and extension as well as lateral movement, but no rotation. The metacarpo-phalangeal and wrist joints are examples of this.

(4) Saddle-shaped joints (Articulus sellaris), allowing the same movements as the last with greater strength. The carpo-metacarpal joint of the thumb is an example.

(5) Ball and socket joints (Enarthrosis), allowing free movement in any direction, as in the shoulder and hip.

(6) Pivot-joint (Trochoides), allowing only rotation round a longitudinal axis, as in the radio-ulnar joints.

Embryology.

Joints are developed in the mesenchyme, or that part of the mesoderm which is not concerned in the formation of the serous cavities. The synarthroses may be looked upon merely as a delay in development, because, as the embryonic tissue of the mesenchyme passes from a fibrous to a bony state, the fibrous tissue may remain along a certain line and so form a suture, or, when chondrification has preceded ossification, the cartilage may remain at a certain place and so form a synchondrosis. The diarthroses represent an arrest of development at an earlier stage, for a part of the original embryonic tissue remains as a plate of round cells, while the neighbouring two rods chondrify and ossify. This plate may become converted into fibro-cartilage, in which