A. Lipoma

This occurs in joints under the form which Johann Müller has distinguished by the name of lipoma arborescens, - a branching growth of fatty tissue in the free part, or in the duplicatures of synovial membranes; it occurs chiefly in the knee-joint.

B. Fibroid Tissue

a. When it occurs in connection with the synovial membrane, it is formed in old exudations; and it adheres to that membrane, assuming some of the manifold shapes already mentioned (p. 175). In some of those forms it accumulates in great abundance. All the loose bodies (or mice, as they are called), which are met with in joints, or at least a great part of them, belong to this class. (Comp. p. 43). They are either masses of exudation set free from the inner surface of the synovial membrane, or growths which have originated beneath it, or, lastly, formations (precipitates) from a synovial fluid of unnatural composition. In the last case, they are distinguished by their laminated structure.

B. Fibroid tissue occurs in the articular ends of the bones in the form of fibroid tumors, which sometimes attain very considerable size.

E. Tubercle

a. Synovial membranes are the seat of tubercle; and, as has been casually remarked already, almost its only form is that in which it is a product of inflammation. It then usually coexists with similar inflamma-ations in large serous membranes, and as both are dependent on advanced tuberculosis of parenchymatous organs, especially of the lungs, it is associated also with a great degree of general tubercular disease. It affects chiefly the larger synovial sacs, such as those of the knee, hip, and elbow. When the exudation softens, and suppuration takes place, the capsule of the joint ulcerates; and secondary tuberculosis, and tubercular ulceration of the bones ensue.

The articular ends of the bones are affected with tubercle sometimes simultaneously with the synovial membrane, and sometimes before it.

. When tubercle is formed in the articular ends of the bones, it is usually a consequence of inflammation, and is diffused through the spongy tissue of the bone. It gives rise to caries, i. e. to tubercular abscesses, which very often break into the cavity of the joint (Nelaton). It is commonly very soon followed by inflammation of the synovial membrane, and tubercular exudation.

Both occasion the most terrible destruction of the tissues of the joint.

D. Cancers

These diseases commence in the articular ends of the bones. The most frequent is medullary cancer, which, after reaching a considerable size, may advance into the cavity of the joint, but never commences, as a primary disease, within or upon the synovial membrane.

6. Anomalies In The Contents Of Synovial Cavities

Most of these may be gathered from what has been said already; and there needs only to mention here, a. Dropsy of the synovial membrane, which arises from the same causes as dropsy of serous sacs in general. In most cases it originates in a slight inflammatory process, the serous product of which contains very little plastic material.

b. The loose bodies, - articular mice, - which have been already described.