Thus far, the process is one of simple infiltration, analogous to the gradual calcification of tissues that takes place in the bodies of the aged. In this way the large ligaments of an affected joint, like the knee and ankle, may in the long course of the disease become trans formed into completely petrified bands and cylinders which only by their position serve to indicate the healthy tissue in which they orig inated. Such utter destruction, however, is not so conspicuous in the capsular ligaments as it is in the large crucial ligaments of the knee and in the structure of the ankle and wrist; and it is only in very inveterate cases that time is afforded for its complete development.
In like manner with the cartilaginous and ligamentous structures, the synovial membranes and their fringe-like processes are invaded. At first, the delicate serous surfaces appear as if dusted over with finely powdered plaster of Paris; but close inspection shows that the deposit is not upon the surface but is laid down in the subserous tissue. This indicates that, as in the cartilages and probably every where, it is an interstitial deposit that takes place instead of a super ficial precipitation of foreign matter. The condition of the syuovial fluid is somewhat variable. In old cases that have suffered severely and have experienced great articular disorganization, the fluid con tents of the joint become turbid, thickened, sometimes acid, and looking like thin mortar. This appearance is due to the presence of urate of sodium in the form of fine particles like dust that can be recovered from the desiccated liquid. But, more frequently, when the cartilages have not been disorganized by ulceration the synovial fluid exhibits little, if any, change. Sometimes, however, there is almost total absence of the lubricating fluid, though the small amount that is present retains its ordinary qualities, and contains .no mineral matter in suspension. The acid reaction of the synovial liquid that has been above noted is not of frequent occurrence.
Even after considerable lesions of the joints it is usual to remark a neutral or alkaline reaction on the part of the articular contents. It is probable that these differences depend chiefly upon the extent to which the joint surfaces have become incrusted with a uratic de posit. So long as that deposit is inconsiderable and does not invade the serous cells that elaborate the synovial fluid, so long will the liquid contents of the joint retain their normal reaction and appear ance. But when the serous membrane has been invaded by the mineral incrustation, as its cells become detached, they carry with them a contribution of sodium orate by which the constitution of the liquid is finally transformed.
By the process above described it is evident that great changes must be effected in the articular structures. But the deposits of min eral matter that are thus formed do not remain inoperative iu the way of farther change. By their presence they interfere mechanically with circulation and nutrition. They serve to irritate the nervous filaments with which the articular tissues are more or less richly en dowed. Consequently, the phenomena of irritation are soon mani fested, and, through the influence upon the nervous system that is thus originated, nutrition is further impaired. Even in mild cases, before the joint surfaces are injured, microscopical examination of the cartilages in which interstitial infiltration has begun shows that inflammatory action is evident in the immediate vicinity of the foreign matter that has been thus intruded into the diarthrodial sub stance.
It is in the cartilage that the process of inflammation begins. Here a double action may be observed : proliferation and necrosis of cartilage cells. In the central portions of the joint the process of necrosis is most conspicuous. Inflammation rapidly leads to cellu lar atrophy, and the uratic deposits are thus laid bare so that by erosion they are worn down, and their substance is washed away into the cavity of the joint. With the extension of the deposit, the pro cess of atrophic inflammation and erosion keeps pace, until the ends of the bones are worn out of all semblance to their original structure. • While this atrophic process gradually erodes the bearings of the joint, a different state of nutrition obtains along the borders of the cartilage. Here.the deposits are relatively small, and the tissues are relatively rich in capillary vessels. Consequently, the inflammatory process results in a more luxuriant cell-growth; the cartilage cells break up and are replaced by embryonal forms ; the marginal struc tures of the articulation become thickened by the growth of what are known as ecchondroses, which are simply masses of proliferating cells originating in the normal cartilages near the insertion of the syno vial membrane, where there is an abundant circulation of nutrient fluids. The free surface of the joint in consequence of this process loses its natural polished appearance, becomes rough and irregular; while the normal elasticity of its fundamental substance is exchanged for a brittle and earthy fragility.