170 



THE ARTICULATIONS OR JOINTS 



^ ^[capsule 



DIARTHROSES 



These joints are characterized b^- the presence of a joint cavity and by their 

 mobility. The}' are often called movable or true joints. The structures which 

 enter into their formation are: 



1. The joint surfaces (Facies articulares), which are usuall}' more or less ex- 

 panded. They are in most cases smooth, and vary much in form. They are 

 formed of specially dense bone, which differs histologically from ordinary compact 

 substance. In certain cases {vide Osteology) the surface is interrupted by non- 

 articular caviti(^s known as s5movial fossae. 



2. The articular cartilages (Cartilagines articulares), usually hyaline in type, 

 form a covering oyer the articular surfaces of the bones. They vary in thickness 

 in different joints; they are thickest on the areas of the greatest pressure, and 

 usually tend to accentuate the curvature of the bone, i. e., on a concave surface 

 the peripheral part is the thickest, while on a convex surface the central part is the 

 thickest. The articular cartilages are non-vascular, very smooth, and have a 



bluish tinge in the fresh state. They diminish the 

 effects of concussion and greatly reduce friction. 



3. The joint capsule (Capsula articularis) is, in 

 its simplest form, a sac, the margins of which are 

 attached around the articulating surfaces. It con- 

 sists of two layers — an external one, composed of 

 fibrous tissue (Stratum fibrosum), and an internal 

 one, the synovial layer or membrane (Stratum syno- 

 viale). The fibrous layer, sometimes termed the 

 capsular ligament, is attached either close to the 

 margins of the articular surfaces or at a variable 

 distance from them. Its thickness varies greatly in 

 different situations: in certain places it is extremely 

 thick, and sometimes cartilage or bone develops in 

 it; in other places it is practically absent, the cap- 

 sule then consisting only of the synovial membrane. 

 Parts of the capsule may undergo thickening and so 

 form ligaments, which are not separable, except 

 artificially, from the rest of the capsule. The syno- 

 vial layer lines the joint cavitj^ except where this 

 is bounded by the articular cartilages; it stops normally at the margin of the latter. 

 It is a thin membrane, and is richly supplied by close networks of vessels and 

 nerves. It frequently forms folds (Plicae synoviales) and villi (Villi synoviales), 

 which project into the cavity of the joint. The folds commonly contain pads of 

 fat which fill up interstices and vary in form and position in various phases of 

 movement. The synovial meml)rane secretes a fluid, the synovia, which resembles 

 white-of-egg and lubricates the joint.^ In many places the membrane forms extra- 

 articular pouches, which facilitate the play of muscles and tendons. 



The joint cavity (Cavum articulare) is inclosed by the synovial membrane and 

 the articular cartilages. Normally, it is, strictly speaking, only a potential cavity, 

 which contains nothing ])ut a small amount of synovia. 



The studont must guard against a false conception of (he joint cavity wliicli may resuh 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 



* It is doubtful whether the sj-novia is a true secretion or a transudate containing protlucts of 

 friction. The view given above is that which is more commonlj^ accepted. 



Fig. 138. — Diagr.\m of Section of 



DiARTHRO.SIS. 



f.l.. Fibrous layer, s.l., synovial 

 layer of joint capsule. The articular 

 cartilages are white, bones dotted, and 

 the joint cavity black in the figure. 



