A joint or articulation is formed where two or more bones of the skeleton meet one another. Where the joints are immovable, as in the articulations of the cranium, the opposed margins of the bones are separated merely by a thin layer of fibrous membrane, named the sutural ligament; in certain regions at the base of the skull this fibrous membrane is replaced by a layer of cartilage. Where slight movement combined with great strength is required, the opposed osseous surfaces are united by tough and elastic fibrocartilages, as in the joints between the vertebral bodies, and in the interpubic articulation. In the freely movable joints the opposed surfaces are completely separated from one another; the parts of the bones forming the articulations are covered with articular cartilage, while the joints are enveloped by articular capsules; and are usually strengthened by strong fibrous bands called ligaments.
In long bones the ends are the parts which form the articulations; they are somewhat enlarged, and consist of spongy substance with a thin coating of compact substance. In the flat bones the articulations usually take place at the edges, and in the short bones at various parts of their surfaces. The layer of compact bone which forms the joint surface, and to which the articular cartilage is attached, is called the articular lamella; its lacunas are large, but it contains no Haversian canals or canaliculi. The vessels of the spongy substance approach the articular lamella but do not perforate it; this lamella is consequently denser and firmer than ordinary bone.
The articular cartilage which covers the articular surfaces of bones, and the fibrocartilage which enters into the structure of some of the joints, are described in the section on Histology. Articular cartilage is usually hyaline in character but, in the case of bones which ossify in membrane, it may be white fibrocartilage.
The ligaments are composed mainly of parallel or closely interlaced bundles of white fibrous tissue, and present a silvery appearance. They are pliant and flexible, so as to allow perfect freedom of movement, but strong, tough, and inextensible, so as not to yield readily to applied force. Some ligaments consist entirely of yellow elastic tissue, as the ligamentum flavum, which connect together the laminae of contiguous vertebrae, and the ligamentum nuchae in the lower animals; the elasticity of such ligaments is intended to act as a substitute for muscular power.
The articular capsules envelop the freely movable joints, and each capsule consists of two strata-an external layer composed of white fibrous tissue; termed the capsular ligament, and an internal layer, termed the synovial membrane of the joint.
The capsular ligament is attached around the ends of the bones entering into the joint, and thus envelops the articulation.
The synovial membrane lines the capsular ligament, and covers those parts of the bones which are within the capsule, but ceases at the margins of the articular cartilages; it is also reflected over any tendons passing through the joint cavity. In some joints the synovial membrane is thrown into folds which pass across the cavity, as in the knee-joint; in others it forms fringe-like processes which generally project from near the margin of the articular cartilage, and lie flat upon its surface. The synovial membrane secretes a small quantity of viscid lubricating fluid, termed synovia.* Histologically, the synovial membrane is composed of delicate, vascular, connective tissue, which is covered on its free surface by an incomplete layer of fattened cells, resembling an endothelium. The cell elements of the tissue are highly phagocytic and migratory. They may be present in the synovia, and they are capable of absorbing micro-organisms and foreign particles injected into the joint. In addition, they probably absorb the cartilaginous debris which must result from ordinary wear and tear.
* Professor E. Barclay-Smith (Proceedings of the Anatomical, Society of.Great Britain and Ireland, Feb. 1922) is of the opinion that an important constituent of this fluid must be derived from the wearing away of the cartilage clad articular surfaces, and suggests that the cartilaginous element in the synovia accounts for its glairy nature and renders it such an ideal lubricant.
Closely related in structure and function to the synovial membrane of the articular capsule, and therefore conveniently defined in this section, are the synovial sheaths (vaginm mucosae) of tendons and the synovial bursa (bursae mucosae).
Synovial sheaths serve to facilitate the gliding of tendons in fibro-osseous canals, e.g. the tendons of the flexor and extensor muscles of the fingers and toes; which pass through canals in or near the hand and foot. Each sheath has the form of an elongated, closed sac, which lines the wall of the canal and is reflected upon the surface of the enclosed tendon or tendons.
Synovial bursa are interposed between surfaces which glide upon each other. They consist of closed sacs containing a minute quantity of clear viscid fluid, and may be grouped, according to their situations, under the headings subcutaneous, submuscular, subfascial, and subtendinous.
In structure the walls of the synovial sheaths and bursm closely resemble the synovial membrane of the articular capsules and, when situated in the neighbourhood of a joint, a subtendinous bursa may be directly continuous with the synovial membrane through a gap in the capsular ligament.