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The Joints

cartilage, joint, bone, bones, cavity, cells and head

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By Joint or Articulation is meant the union of two bones by means of other structures. The structures that enter into the formation of a joint are : (1) bone, (2) cartilage or gristle, (3) synovia, a smooth delicate membrane which lines all parts of the inside of the joint except the opposing surfaces of cartilage, (4) liga ments, strong bands to bind the bones to gether.

Bone has already been described (p. 57).

Hyaline cartilage consists of a ground sub stance of a fine ground-glass appearance, con taining cells, which have a nucleus and nucleolus, and lie in spaces in the ground substance iticlosed by a capsule. One cell may till one space, but oftener two, three, four, or more cells are pre sent, which have obviously originated by divi sion from one original cell (Fig. 28).

In white fibro-cartilage the ground-sub stance is everywhere pervaded, and almost replaced, by white fibrous tissue, disposed in layers of bundles, the encapsuled cartilage cells lying between the bundles.

In yellow fibro-cartilage yellow elastic fibres form a basket-work, the cartilage cells occupy ing the meshes.

The first variety is found coating the opposing ends of bones, entering into the formation of a joint (articular cartilage), forming the portions of the ribs that become attached to the breast bone (costal cartilage), and it is this kind of cartilage that occupies the place of bone in foetal life, being afterwards replaced by bone. White fibro-cartilage is found in the discs be tween the vertebrae of the spinal column ; and the third variety forms the main portion of the lid of the windpipe—the epiglottis, and of other cartilages of the voice-box or larynx (p. 354).

Cartilage is tough but highly elastic. Dis posed between bones it acts as a buffer, while permitting a certain amount of flexibility.

This is a very delicate tissue, and frcm it a fluid, synovia, is poured out to moisten the cavity of the joint and so reduce the amount of friction and heat developed by movement. It is formed of a single layer of cells resting on a connective-tissue basis, and is richly supplied with blood-vessels.

These are connecting bands made of bundles of delicate wavy fibres bound firmly together.

They are not elastic. They pass from one bone to another, strongly supporting the joint, which sometimes they completely surround.

In a joint, then, you have the ends of two opposed bones, of which the opposing surfaces are coated with cartilage; strong ligaments pass between them to complete and maintain the union ; and the inner surface of the joint cavity is lined by a membrane which pours out a fluid to lubricate the joint.

It is, however, only perfect joints that are thus fully equipped, and there are joints which want one or other or several of these structures, which are therefore called incomplete or im perfect joints.

Imperfect Joints. The bones of the skull are united by their ragged or serrated edges being dovetailed into one another, no structures intervening between the bones. Such joints are called sutures and are immovable.

In the union effected between the bodies of the vertebrae there is an example of incom plete joints, which are partially movable. Be tween two opposed bodies of vertebra: there is a pad or cushion of cartilage of the white fibrous kind. The pads are elastic and useful in pre venting jars to the vertebral column. Besides, they allow of a considerable amount of move ment over all, though very little between any two vertebrae. The union is strengthened by ligaments, but there is no synovial membrane.

Perfect Joints. There are various hams, according to the nature and amount of the movement permitted.

Ball-and-socket Joints. In this form one bone has a cup-like depression into which the head of the other fits. This is the kind of joint existing between the head of the arm bone and the glenoid cavity (p. 62) of the shoulder-blade, and between the head of the thigh-bone and the acetabulnin of the Miami nate bone. In the hip-joint the head of the bone is kept close in the cavity by means of a special ligament within the cavity itself, the round ligament, which passes from a depres sion in the bottom of the acetabulum to the head of the thigh-bone. Ball-and-socket joints permit free motion in almost any direction.

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