DIARTHROSES 209 
membrane forms extra-articular pouches, which facilitate the play of muscles and 
tendons. 
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. 
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 sec- 
tions. On the other hand, it is 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, 7. 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 
dises 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 
nuche, 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 ligaments (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 
fascie function as ligaments 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 lymph- 
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 vertebre. 
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 
