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 direction. 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 per fect example, because in certain positions it is capable of a slight rotation. These hinge joints are always provided with strong lateral ligaments. The shells of bivalve mollusks are united by a very strong and perfect hinge-joint.
The last kind of joint requiring notice is that which admits only of rotatory motion. A pivot and a 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.—Formerly all the severer forms of diseases of the joints were vaguely designated under the one general term uhite swelling; but during the last half' century, thanks to the labors of modern surgeons, amongst whom the name of the late sir Benjamin Brodie stands prominently conspicuous, the diseases of the joints are tolerably well understood, and can be discriminated from one another with very con siderable accuracy.
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 MEM BRANES AND FLuin).
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. When they get between the ends of the bones, which they are apt to do during exercise, they cause a sudden and often a most excruciating pain, which is often followed_ by inflammation, and arrest all motion of the joint. These symptoms are not relieved
till, by gentle flexion and manipulation, the loose cartilage (as it is usually termed) has been removed to a position in which it ceases to give annoyance. When the displace ment of the loose body is only occasional, and does not cause intensely severe pain, the treatment should be limited to the application of an elastic bandage or a tightly fitting knee-cap, which should be constantly worn, with the view of restraining the loose body to a position in which it is inoffensive. If, however, this palliative treatment fails, the offending body must be removed by subcutaneous incision, which avoids the danger of a direct wound into the joint.
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, hut not always, begin with the disorganization of carti lage, and then extend to the bones. Sometimes, however, they commence in the bones. The consideration of the symptoms and general treatment of these diseases would be out of place in these pages, but a reference to one very important mode of treating articular caries will be found in the article RESECTION or JOINTS. Several of the preced ing diseases, even when the result of our treatment may be regarded as satisfactory, leave a certain amount of stiffness of the joint (sometimes extending to perfect immo bility), to which the term ankylosis (q.v.) is applied.