[Illustration: 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.] [Illustration: 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.