208 THE ARTICULATIONS 
(5) Gomphosis.—This term is sometimes applied to the implantation of the 
teeth in the alveoli. 
The gomphosis is not, properly considered, a joint at all, since the teeth are not parts of the 
skeleton. 
DIARTHROSES 
These joints are characterized by the presence of a joint cavity and by their 
mobility. They are often called movable or true joints. A simple joint (Articu- 
latio simplex) is one formed by two articular surfaces; a composite joint (Articulatio 
composita), one formed by several articular surfaces. The following structures 
enter into their formation: 
1. The articular surfaces (Facies articulares) are in most cases smooth, and 
vary much in form. They are formed of specially dense bone, which differs his- 
tologically from ordinary compact substance. In certain cases (vide Osteology) 
the surface is interrupted by non-articular cavities known as synovial fosse. 
2. The articular cartilages (Cartilagines articulares), usually hyaline in type, 
form a covering over the articular surfaces of the bones. They vary in thick- 
ness in different joints; they are thickest on those 
which are subject to the most pressure and fric- 
tion. They usually tend to accentuate the curva- 
ture of the bone, 7. e., on a coneave surface the 
peripheral part is the thickest, while on a con- 
Fe vex surface the central part is the thickest. The 
ies 1 articular cartilages are non-vascular, very smooth, 
sl) . 
and have a bluish tinge in the fresh state. They 
diminish the effects of concussion and greatly reduce 
friction. 
3. The articular or joint capsule (Capsula articu- 
laris) is, nits simplest form, a tube, the ends of which 
are attached around the articulating surfaces. It 
consists of two layers—an external one, composed of 
Fic. 
229.—D1aGrRaM oF SECTION OF 
DIARTHROSIS. 
f.., Fibrous layer, s.l., 
layer of joint capsule. The articular 
cartilages are white, bones dotted, and 
the joint cavity black in the figure. 
synovial 
fibrous tissue, and an internal one, the synovial layer 
ormembrane. ‘The fibrous layer (Stratum fibrosum), 
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. Tendons which pass over a 
joint may partially take the place of the fibrous layer; in these cases the deep face 
of the tendon is covered by the synovial layer. Parts of the capsule may undergo 
thickening and so form ligaments, which are not separable, except artificially, from 
the rest of the capsule. The synovial layer (Stratum synoviale) lines the joint 
‘avity 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 (Plicze synoviales) and 
villi (Villi synoviales), which project into the cavity of the joint. The folds com- 
monly contain pads of fat, and there are in many places masses of fat outside of the 
capsule which fill up interstices and vary in form and position in various phases of 
movement. The synovial membrane secretes a fluid, the synovia, which lubricates 
the joint; it resembles white-of-egg, but has a yellowish tinge.!. In many places the 
1Tt is doubtful whether the synovia is a true secretion or a transudate containing products of 
friction. The view given above is that which is more commonly accepted. It contains albumen, 
mucin, and salts, and is alkaline. In it there are commonly cells derived from the synovial mem- 
brane, portions of cells, cells which have undergone fatty degeneration, particles of articular 
cartilage, ete. 
