The atrophy, or absorption of cartilage, is so frequently seen in the joints of old subjects, that some authors have been disposed to regard it rather as a physiological than a pathological condition. It is, however, unquestionably a disease; and in the drawings above mentioned, are some specimens of it from a soldier under the middle age. The cartilage seems in the first instance to be opened out in its texture, and numerous little villous processes appear on its surface, often as if its fibres were enlarged after absorption of the substance which connected them. The thinning sometimes takes place in patches, occasionally in grooves, corresponding to the motions of the articular surfaces; the subjacent bone becomes at length completely denuded, and soon presents a polished porcelain-like surface, which is so remarkable that it is commonly known under the name of porcellaneous deposit. It is obvious that, when the disease has proceeded thus far, the joint must become more or less deformed, and its motions considerably impeded. Accordingly, old men are often seen at work with much rigidity of the joints, particularly of the knee, which is frequently projected inwards, from absorption of the cartilage from the outer condyle of the femur and corresponding part of the tibia, with a thinning and degeneration of the inner-articular cartilage into a tissue resembling the cellular. The hard, polished, and ivory-like appearance of the articular surface, would lead us to suppose that it contained an unusual quantity of earthy matter, yet an analysis by Dr. Davy, here given, shows the contrary. _Composition of the Shaft._ _Composition of the polished Articular Surface._ Phosphate of lime, &c. 58·8 Phosphate of lime 54·2 Animal matter 41·2 Animal matter 45·8 ——-—— ————- 100·0 100·0 In many instances where the disease is of long standing, a deposit of adventitious bone takes place around the articular surfaces, and this may occur to such an extent as to produce anchylosis of the joint, the articular extremities of the bone often presenting several irregular depressions, and becoming variously altered in shapes, as shown in the cuts, p. 86. It is curious that the atrophy of cartilage is generally seen in its early stage to attack the joints in pairs, and to occur also in corresponding parts of the articular surfaces; a fact which coincides with the disposition of caries to appear simultaneously or successively in corresponding molar teeth.