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. Thej' 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 sxirfaces (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 kno^^m as synovial fossae. 



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, i. e., on a concave surface the 

 peripheral part is the thickest, while on a con- 

 vex 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 articular or joint capsule (Capsula articu- 

 laris) is, in its simplest form, a tube, the ends of which 

 are attached aroimd the articulating surfaces. It 

 consists of two layers — an external one, composed of 

 fibrous tissue, and an internal one, the synovial layer 

 or membrane. 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 

 cavity 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 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 



' It 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 S3nQovial mem- 

 brane, portions of cells, cells which have undergone fatty degeneration, particles of articular 

 cartilage, etc. 



Fig. 229. — Diagram of Section of 



DiARTHROSIS. 



f.l.. Fibrous layer, &.I., synovial 

 layer of joint capsule. The articular 

 cartilages are white, bones dotted, and 

 the joint cavity black in the figure. 



