The mechanical axiom that "what is gained in movement is lost in stability" applies to joints, those which have the widest range of movement being the most frequently dislocated. * * * * * The injuries to which a joint is liable are Contusions, Wounds, Sprains, and Dislocations. #Contusions of Joints.#--Contusion is the mildest form of injury to a joint. Whether the violence is transmitted from a distance, as in contusion of the hip from a fall on the feet, or acts more directly, as in a fall on the great trochanter, the bones are violently driven against one another, and the force expends itself on their articular surfaces. The articular cartilages and the underlying spongy bone, as well as the synovial lining, are bruised, and there is an effusion of blood and serous fluid into the joint and surrounding tissues. The most prominent _clinical features_ are swelling and discoloration. The swelling, especially in superficially placed joints, is an early and marked symptom, and is mainly due to the effusion of blood into the joint (_hæmarthrosis_). In deeply placed joints, discoloration may not appear on the surface for some days, especially if the violence has been indirect. The joint is kept in the flexed position, and is painful only when moved. In hæmophilic subjects, considerable effusion of blood into a joint may follow the most trivial injury. A slight degree of serous effusion into the joint (_hydrarthrosis_) often persists for some time, and tuberculous affections of joints not infrequently date from a contusion. The _treatment_ is the same as for sprains (p. 36). #Wounds of Joints.#--The importance of accidental wounds of joints--such, for example, as result from a stab with a penknife or the spike of a railing--lies in the fact that they are liable to be followed by infection of the synovial cavity. The infection may involve only the synovial layer (_septic synovitis_), or may spread to all the elements of the joint (_septic arthritis_). These conditions are described with diseases of joints. Penetration of the joint may sometimes be recognised by the escape of synovia from the wound, or the synovial layer or articular cartilage may be exposed. When doubt exists, the wound should be enlarged. The use of the probe is to be avoided, on account of the risk of carrying infective material from the track of the wound into the joint.