Joints, in Anatomy. A joint or articulation may be defined to be the union of any two segments of the skeleton of an animal body, through the intervention of a structure or structures of a different nature. The textures which enter into the formation of the more complex joints are bone, cartilage, fibro-cartilage, ligaments, and synovial membrane. Bone forms the fundamental part of all joints; ligament, in various modifications, is employed as the bond of union between the bony segments; while the three remaining textures chiefly occur in those joints in which there is free motion. The joints vary in the degree of motion from almost perfect immobility to the greatest amount and extent of motion that are compatible with the maintenance of the bony segments in their proper relation to each other.
Joints have been divided by anatomists into two great classes—the Immovable and the Movable. In the immovable or Synarthroses the parts are continuous, that is to say the bones are united together by a prolongation of the periosteal fibrous membrane between them. In some cases the uniting medium is a plate of cartilage. There is no synovial sac intervening between the bones. In movable joints the articular surface of each of the bones is covered with cartilage, and these cartilaginous plates are separated from each other by a synovial sac more or less complete. This sac is lined by a membrane which secretes a viscid fluid for lubricating the articular surfaces—the synovia or joint-oil.
In synarthroses the articulation is said to be by suture when the bones seem to grow somewhat into one another, and to become interlocked and dovetailed together, each bone having a jagged or serrated margin, or when there is a degree of beveling, so that one bone is overlapped by the other. Both these kinds of suture are at once seen in the human Skull (q.v.).
The movable joints are Amphiarthroses and Diarthroses. In the former there is partial mobility of one bone upon another, combined with great strength. The contiguous surfaces of the bones are united by a thick and strong layer of fibro-cartilage, the centre of which is usually soft, and may present a cavity lined by a synovial membrane, with which a little elastic tissue is intermixed. As examples of this kind of joint may be mentioned the articulation between the bodies of the vertebrae and that between the two pubic bones at what is termed the symphysis.
Diarthroses are complete joints, the articular surfaces being covered by articular cartilage and separated from each other by a cavity lined by synovial membrane. In these the degree and nature of the motion are very various. There may be merely a little gliding motion between the ends of the bones, as, for example, in the articulations between the various bones of the carpus and tarsus. (see HAND, FOOT). In these cases the surfaces are plane, or one is slightly concave and the other slightly convex; and the motion is limited in extent and direction by the ligaments of the joint, or by some projecting point of one of the bones. In some cases, instead of a slight concavity and convexity, one bone presents a cup-like depression, while the termination of the other assumes a hemispherical, or more or less globular shape. Hence the name of ball and socket that is applied to such joints. The best example of this variety is the Hip-joint (q.v.), and the next best is the shoulder. In these joints the ball is kept in apposition with the socket by means of what is termed a capsular ligament, which may be described as a barrel-shaped expansion of ligamentous structure, attached by its extremities around the margin of the articular surfaces composing the joint, and forming a complete investment of it, but not so tight as materially to restrict its movements. This species of joint is capable of motion of all kinds.
Another important variety of articulation is the hinge-joint, in which the contiguous surfaces are marked with elevations and depressions, which exactly fit into each other, so as to restrict motion to one plane. The elbow and ankle joints, and the joints of the fingers and toes, are the best examples of this variety. The knee-joint is a less characteristic example, because in certain positions it is capable of a slight rotation. These hinge-joints are always provided with strong lateral ligaments.
The last kind of joint requiring notice is that which admits only of rotatory motion. A pivot and a ring are the essential parts of this joint, the ring being generally formed partly of bone and partly of ligament. The best example of this articulation is that between the atlas (the first vertebra) and the odontoid or tooth-like process of the axis (the second vertebra). See HAND.
Diseases of the Joints.—In diseases of the joints we may have one or more of the following textures affected: (1) the synovial membrane; (2) the cartilage; and (3) the bones themselves. The synovial membrane may undergo either acute or chronic inflammation, giving rise to the serious affections known as acute and chronic Synovitis (see SYNOVIAL MEMBRANES). Loose substances of a fibrous structure, and usually resembling a small bean in size and shape, sometimes occur in joints, especially in the knee-joint. They commence as little pendulous growths upon the synovial membrane, which after a time become detached. The cartilage may be affected in various ways. There may be (1) simple destruction of cartilage; (2) scrofulous destruction of cartilage; (3) hypertrophy of cartilage; (4) atrophy of cartilage, and other modified forms of disease of this texture, all of which, especially the second, are of a very serious character, but not of a nature that admits of popular explanation. The most important diseases of the osseous structures of the joints are (1) ulcer and (2) caries. These diseases often, but not always, begin with the disorganisation of cartilage, and then extend to the bones. Sometimes, however, they commence in the bones. See ANKYLOSIS.
Resection or Excision of Joints 'is on the whole safer than amputation; less violence is done to the body, fewer great arteries and nerves are injured, and, what is of more consequence, fewer large veins are divided, and as the articular end of the bone only is sawn off, and the medullary canal not touched, there is less chance of pyæmia. Lastly, the patient is left with an imperfect limb, it is true, but with one which, in most cases, is highly useful' (Druitt): The operation has been performed on the shoulder, elbow, wrist, hip, knee, and ankle. Few subjects have in recent times excited more discussion among surgeons than the application of this operation to the knee-joint. The operation was first performed in 1762; and up to the year 1830 there are records of 19 cases, out of which 11 died. From 1830 to 1850 the operation was never performed, and was generally condemned; but in the last-named year it was revived by Professor Fergusson, and is now a frequent and most valuable operation. 'The cases,' says Dr Druitt, 'in which it ought to be performed are, generally speaking, such cases of injury or disease as would otherwise be submitted to amputation. The object of the operation is to produce a firm and useful limb, slightly shortened, and with entire bony union, or fibrous union admitting of some small degree of motion at the situation of the joint. But all cases are not suitable for excision; and those cases are unsuitable and better adapted for amputation in which either the quantity of the diseased bone is very great, or the quality of the disease may be such as experience has shown to be incompatible with the exudation of healthy material of repair.' In at least 50 per cent. of cases the operation results in a good useful leg. It has already saved so many limbs that it must be regarded as one of the greatest triumphs of modern surgery. Further information on this subject may be found in Holmes's System of Surgery, or in any surgical text-book—e.g. Erichsen's.
Joints, in Geology, are the natural division-planes or cracks by which rocks of all kinds are traversed. Joints, although very frequently irregular, yet have a tendency to run across rocks in certain directions. Thus, in ordinary bedded aqueous rocks (sandstone, shale, limestone) they are generally developed more or less at right angles to the bedding, so that, if the strata be horizontal, the leading joints will be vertical or approximately so. Two sets of these joints are usually recognisable (master-joints), which cut each other at or nearly at right angles. Hence aqueous rocks, by means of joints and original bedding-planes, are divided into larger or smaller cuboidal blocks. In massive crystalline rocks, such as granite, joints are rarely so regular. Yet even in these two sets of joints, crossing each other, can often be traced: and occasionally another horizontal set may be present—these last simulating the bedding-planes of aqueous strata. Were it not for the presence of such natural division-planes, it is obvious that quarrying would be a much more difficult operation. A peculiar kind of jointing is met with in certain crystalline igneous rocks, as in some fine-grained basalts, in which the division-planes separate the rock into polygonal or more or less perfect hexagonal prisms or columns (see BASALT). Joints have been formed in various ways. Many are doubtless due to the strain and tension to which rocks have been subjected during movements of the crust. Others probably owe their origin to contraction on cooling: the prismatic joints of basalt being ‘fissures of retreat.’ And, in like manner, it seems likely enough that sedimentary strata may sometimes have become jointed during their gradual drying and consolidation.