The section devoted to the Articulations or Joints deals with the union of the various and dissimilar parts of the human skeleton. The followiing structures enter into the formation of joints. Bones constitute the basis of most joints. The long bones articulate by their ends, the flat by their edges, and the short at various parts on their surfaces. The articular ends are usually expanded, and are composed of cancellous tissue, surrounded by a dense and strong shell of compact tissue.
This shell has no Haversian canals (the vessels of the cancellous tissue turn back and do not perforate it), or large lacunae, and no canaliculi, and is thus well adapted to bear pressure. This "osteoid" layer may represent in part calcified cartilage rather than true bone.
The cartilage which covers the articular ends of the bones is called articular, and is of the hyaline variety. It is firmly implanted on the bone by one surface, while the other is smooth, polished, and free, thus reducing friction to a minimum, while its slight elasticity tends to break jars. It ends abruptly at the edge of the articulation, and is thickest over the areas of greatest pressure.
Another form of cartilage, the white fibrous, is also found in joints:
- As interarlicular cartilage in diarthrodial joints - viz., in the knee, mandibular, sterno-clavicular, radio-carpal, and occasionally in the acromio-clavicular joint. It is interposed between the ends of the bones, partially or completely dividing the synovial cavity into two. It serves to adjust dissimilar bony surfaces, adding to the security of, while it increases the extent of motion at, the joint; it also acts as a buffer to break shocks.
- As circumferential or marginal fibro-cartilages, which serve to deepen the sockets for the reception of the heads of bones - e. g., the glenoid ligaments of the shoulder and hip. Another form of marginal plate is seen in the accessory volar ligaments of the fingers and toes, which deepen the articulations of the phalanges and add to their security.
- As connecting fibro-cartilage. The more pliant and elastic is the more cellular form, and is found in the intervertebral discs; while the less yielding and more fibrous form is seen in the sacro-iliac and pubic articulations, where there is little or no movement.
The ligaments which bind the bones together are strong bands of white fibrous tissue, forming a more or less perfect capsule [capsula articularis], round the articulation. They are pliant but inextensile, varying in shape, strength, and thickness according to the kind of articulation into which they enter. They are closely connected with the periosteum of the bones they unite. In some cases - as the ligamenta flava which unite parts not in contact - they are formed of jellow elastic tissue.
The synovial membrane [stratum synoviale] lines the interior of the fibrous ligaments, thus excluding them, as well as the cushions or pads of fatty tissue situate within and the tendons which perforate the fibrous capsule, from the articular cavity. It is a thin, delicate membrane, frequently forming folds and fringes which project into the cavity of the joint; or, as in the knee, stretches across the cavity, forming a so-called synovial ligament. In these folds are often found pads of fatty tissue, which fill up interstices, and form soft cushions between the contiguous bones. The amount of fat that is normally present within a joint varies greatly. It is an old observation that although there is always fat in the hip-and knee-joints, there is usually none within the shoulder-joint. Sometimes these fringes become villous and pedunculated, and cause pain on movement of the joints. They contain fibro-fatty tissue, with an isolated cartilage cell or two. The synovial membrane is well supplied with blood, especially near the margins of the articular cartilages and in the fringes. It secretes a thick, glairy fluid like white of egg, called synovia, which lubricates the joint. Another variety of synovial membrane is seen in the bursas, which are interposed between various moving surfaces. In some instances bursas in the neighbourhood of a joint may communicate with the synovial cavity of that joint.