Articular system
A joint or articulation (or articular surface) is the location at which bones connect (see skeletal system). Articulations can be divided into fixed, semi-mobile and mobile, which are the most important for the movement. The bones connected by mobile joints at the end points (articular ends) are covered by a layer of smooth and elastic cartilage tissue, which allows the sliding between the contact surfaces without friction. In turn, the articular cartilages are covered by a thin membrane that during the movement produces the synovial fluid lubricating the joint and absorbed during rest. This membrane forms the inner lining of the fibrous capsule that surrounds the joint and it is used to create a more solid union between the two bones. Often, the capsule is strengthened by ligaments that are meant to keep the joint firm and to allow a correct position during movement. In general, ligaments are external, but they can also be internal, as for example those in the knee. Often, tendons contribute to the outer reinforcement of the joint, as they are the ends of the muscles inserted on the articulating bones. When the particular heads are irregularly shaped, there are small discs of cartilage interposing with bearings: the menisci.
