rs of the Lyons school regard arthritis
deformans as due to an attenuated form of tuberculous infection, and
draw attention to the fact that a tuberculous family history is often
met with in the subjects of the disease.

[Illustration: FIG. 158.--Arthritis Deformans of Knee, showing
eburnation and grooving of articular surfaces.

(Anatomical Museum, University of Edinburgh.)]

_Morbid Anatomy._--The commonest type is that in which the articular
surfaces undergo degenerative changes. The primary change involves the
articular cartilage, which becomes softened and fibrillated and is worn
away until the subjacent bone is exposed. If the bone is rarefied, the
enlarged cancellous spaces are opened into and an eroded and worm-eaten
appearance is brought about; with further use of the joint, the bone is
worn away, so that in a ball-and-socket joint like the hip, the head of
the femur and the acetabulum are markedly altered in size and shape.
More commonly, the bone exposed as a result of disappearance of the
cartilage is denser than normal, and under the influence of the
movements of the joint, becomes smooth and polished--a change described
as _eburnation_ of the articular surfaces (Fig. 158). In hinge-joints
such as the knee and elbow, the influence of movement is shown by a
series of parallel grooves corresponding to the lines of friction
(Fig. 158).

[Illustration: FIG. 159.--Hypertrophied Fringes of Synovial Membrane in
Arthritis Deformans of Knee.

(Museum of Royal College of Surgeons, Edinburgh.)]

While these degenerative changes are gradually causing destruction of
the articular surfaces, reparative and hypertrophic changes are taking
place at the periphery. Along the line of the junction between the
cartilage and synovial membrane, the proliferation of tissue leads to
the formation of nodules or masses of cartilage--_ecchondroses_--which
are subsequently converted into bone (Fig. 157). Gross alterations in
the ends of the bone are thus brought about which can be recognised
clinically and in skiagrams, and which tend to restrict the normal range
of movement. The extension of the ossification into the synovial
reflection and capsular ligament adds a collar or "lip" of new bone,
known as "lipping" of the articular margins, and also into other
ligaments, insertions of tendons and intermuscular septa giving rise to
bony outgrowths or osteophytes not unlike those met with in the
neuro-arthropathies.

Proliferative