250 THE ARTICULATIONS OR JOINTS. 
articulating bones together, but to a large extent they form the peripheral 
boundary of the joint cavity, although not equally developed in all positions. 
Thus every diarthrodial joint possesses a fibrous or ligamentous envelope or 
capsule which is attached to the adjacent ends of the articulating bones. For 
special purposes, particular parts of the capsule may undergo enlargement and 
thickening, and so constitute strong ligamentous bands, although still forming con- 
tinuous constituents of the capsule. 
Within the capsule a series of intracapsular structures are present. Thus the 
capsule itself is always lined by a synovial membrane, which is continued from the 
inner surface of the capsule to the surface of the intracapsular portion of each 
articulating bone. The part of the bone included within the capsule consists of a 
“non-articular” portion covered by synovial membrane, and an “articular” portion 
covered by encrusting hyaline cartilage. The latter provides the surface which 
comes into apposition with the corresponding area of another bone. In its general 
disposition the synovial membrane may be likened to a cylindrical tube open at 
each end. 
Certain diarthroses present additional intracapsular structures which may be 
distinguished as interarticular ligaments and interarticular fibro-cartilages. 
Interarticular ligaments extend between and are attached to non-articular areas 
of the intracapsular portions of the articulating bones. They usually occupy the 
long axis of the joint, and occasionally they widen sufficiently to form partitions 
which divide the joint-cavity into two compartments, eg. the costo-central, and — 
certain of the chondro-sternal joints. 
Interarticular fibro-cartilages or menisci (Fig. 197) are more or less complete 
partitions situated between and separating opposing articular surfaces, and when — 
complete they divide the joint cavity into two distinct compartments. By its 
periphery, a meniscus is rather to be associated 
Artienar With the joint capsule than with the articu- — 
\ Vane: lating bones, although its attachments may — 
\ extend to non-articular areas on the latter. 
Both interarticular ligaments and menisci 
ae have their free surfaces covered by synovial i 
3 
. 
; 
Synovial 
membrane — nite membrane. 
Adipose tissue forming pads of varying size 
is usually found in certain localities within 
the joint, between the synovial membrane and 
Articnlar Ue surfaces which it covers. These pads are 
cartilage soft and pliable, and act as packing material, 
filling up gaps or intervals in the joint. 
During movement they adapt themselves to 
Fic. 197.—Dracram or a Diarruropia Jorr the changing conditions of the articulation. 
WITH AN INTERARTICULAR MeENiscus Divip- In addition to merely binding together twa 
Pe ee gy INE: Cavity TNO TWO COM GO» more articulating bones, ligaments perform 
very important functions in connexion with 
the different movements taking place at a joint. They do not appreciably lengthen 
under strains, and thus ligaments may act as inhibitory structures, and by becom- 
Ing tense may restrain or check movement in certain directions. 
Meniscus 
Synovial 
membrane — 
Joint 
cavity 
Joint 
capsule 
Synovial membranes, in the form of closed sacs termed synovial bursa, are frequently found 
in other situations besides the interior of joints. Such burs are developed for the purpose of 
reducing the friction, («) between the integument and certain prominent subcutaneous bony pro- 
jections, as, for instance, the point of the elbow, or the front surface of the patella (subcutaneous 
synovial bursee) ; (b) between a tendon and some surface, bony or cartilaginous, over which it 
plays (subtendinous synovial bursee) ; (c) between a tendon or a group of tendons and the walls 
of osteo-fascial tunnels, in which they play (thecal synovial bursie). Subtendinous synovial 
burs are often placed in the neighbourhood of joints, and in such cases it not infrequently 
happens that there is a direct continuity between the bursa and the synovial membrane which 
lines the cavity of the joint through an aperture in the joint-capsule. 
