DIARTHROSES 209 



Sndo^ns''''^ ^''^^ extra-articulat pouches, which facilitate the play of muscles and 



The articular or joint cavity (Cavum articulare) is enclosed by the synovial 

 membrane and the articular cartilages. Normally, it is, strictly speaking, only a 

 potential cavity, which contains nothing but a small amount of synovia. 

 A\..^J^^ student must guard against a false conception of the joint cavity which mav result from 

 tT.^°T ^^d diagrams m which an actual cavity of considerable extent appeSs to e^st A 

 tTonf Jdea of the intimate apposition of the parts is best obtained from the sZdyo frozen' sec^ 

 H?«?^;h ?>, ° w"" hand, it IS instructive to examine joints which have been injected so as to 



distend the capsule fuUy. It is then seen that the cavity is often of much greater Potential ext en? 



saccTtioTs' '"P^°''' ^""^ *'^"' *^' "^'P'"^^ '^ °'*^° ^^"-y '"^^"'^ '^ form'i Mo?msf;Sy of 



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 (Ligamenta) are strong bands or membranes, usually 

 composed of white fibrous tissue, which bind the bones together. They are pli- 

 able but practically inelastic. In a few cases, however, e. g., the ligamentum 

 nucha, they are composed of elastic tissue. They may be subdivided, according 

 to position, into periarticular and intraarticular. Periarticular ligaments are fre- 

 quently blended with or form part of the fibrous capsule; in other cases they are 

 quite distinct. Those which are situated on the sides of a joint are termed col- 

 lateral Ugaments (Ligamenta collateralia) . Strictly speaking, intra-articular liga- 

 ments, though within the fibrous capsule, are not in the joint cavity; the synovial 

 membrane is reflected over them. The term seems justifiable, however, on prac- 

 tical grounds. ^ Those which connect directly opposed surfaces of bones are termed 

 interosseous ligaments. In many places muscles, tendons, and thickenings of the 

 fascia; function as hgaments and increase the security of the joint. Atmospheric 

 pressure and cohesion play a considerable part in keeping the joint surfaces in appo- 

 sition. 



5. Articular discs or menisci (Disci s. menisci articulares) 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 cer- 

 tain surfaces congruent 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 pre- 

 vent fracture of the margin. 



Vessels and Nerves. — The arteries form anastomoses around the larger joints, 

 and give off branches to the extremities of the bones and to the joint capsule. The 

 synovial membrane has a close-meshed network of capillaries; the latter form loops 

 around the margins of the articular cartilages, but do not usually enter them. 

 The veins form plexuses. The synovial membrane is also well supplied with l3rmph- 

 vessels. Nerve-fibers are especially numerous in and around the synovial mem- 

 brane and there are special nerve-endings, e. g., Pacinian bodies and the articular 

 end-bulbs described by Krause. 



Movements. — The movements of a joint are determined chiefly by the form 

 and extent of the joint surfaces and the arrangement of the ligaments. They are 

 usually classified as follows: 



1. Gliding. — This refers to the sliding of one practically plane surface on 

 another, as in the joints between the articular processes of the cervical vertebrae. 



2. Angular Movements. — In these cases there is movement around one or 

 more axes. Motion which diminishes the angle included by the segments forming 

 the joint is termed flexion, while that which tends to bring the segments into line 

 with each other is called extension. With reference to the joints of the distal parts 



14 



