The student must guard against a false conception of the joint cavity which may result from dissections and diagrams, in which an actual cavity of considerable extent appears to exist. A correct idea of the intimate apposition of the parts is best obtained from the study of frozen sections. It is also instructive to examine joints which have been injected so as to distend the capsule fully. It is then seen that the cavity is often of much greater potential extent than one might suppose, and that the capsule is often very irregular in form, _i. e._, forms a variety of sacculations. The foregoing are constant and necessary features in all diarthroses. Other structures which enter into the formation of these joints are ligaments, articular discs or menisci, and marginal cartilages. 4. =Ligaments.=—These are strong bands or membranes, usually composed of white, fibrous tissue, which bind the bones together. They are pliable, but practically inelastic. In a few cases, however, _e. g._, ligamenta flava, ligamentum nuchæ, they are composed of elastic tissue. They may be subdivided, according to position, into =periarticular= and =intraarticular=. Periarticular ligaments are frequently blended with or form part of the fibrous capsule; in other cases they are quite distinct. Strictly speaking, intraarticular ligaments, though within the fibrous capsule, are not in the joint cavity; the synovial membrane is reflected over them. The term seems justifiable, however, on practical grounds. Ligaments which connect directly opposed surfaces of bones are termed interosseous. The special names are based usually on their position, form, and attachments, _e. g._, lateral or collateral, cruciate, sacro-iliac, etc. In many places muscles, tendons, and thickenings of the fasciæ function as ligaments and increase the security of the joint. Atmospheric pressure and cohesion play a considerable part in keeping the joint surfaces in apposition. 5. =Articular discs= or =menisci= are plates of fibro-cartilage or dense fibrous tissue placed between the articular cartilages, and divide the joint cavity partially or completely into two compartments. They render certain surfaces congruent, _e. g._, femoro-tibial joint, allow greater range or variety of movement, and diminish concussion. 6. A =marginal cartilage= (Labrum glenoidale) is a ring of fibro-cartilage which encircles the rim of an articular cavity. It enlarges the cavity and tends to prevent fracture of the margin.