The articular cartilages and the fibro-cartilages may be the seat of congestion, with ramified or uniform redness, and areas of swelling, softening, absorption, erosion and ulceration, the ulcers varying in size from a millet seed upward. The nuclei increase in size and the cartilage cells multiply. In chronic forms calcification is not uncommon.
The articular lamella and subjacent bone may show inflammation with increased vascularity, softening and even ulceration. In the chronic forms on the parts denuded of cartilage, the surface of the bone may show the clear, polished condition known as eburnation.
The binding ligaments may show congestion, thickening, exudation, softening, with increased rigidity, or in some cases relaxation. Calcification is not uncommon in chronic cases. The soft parts around the joints are often extensively infiltrated and swollen.
In horses these lesions are specially common in the fetlock, but occur also in the knee, elbow, shoulder, hips, stifle and hock. In cattle they tend to attack the same parts with a preference perhaps for the hocks and fetlocks.
Changes in the blood. The blood becomes profoundly altered, the albumen reduced, the fibrinogenous elements increased (5 to 10 parts per 1000 instead of three), the red blood globules disappear (in man 2,850,000 per cubic millimeter instead of 4,500,000), the hæmoglobin and oxidizing power of the blood are reduced by about 50 per cent., the leucocytes are relatively increased, and coagulation takes place with unusual firmness, a cupped surface and an excess of buffy coat. No excess of urea, uric acid, nor lactic acid, has been found, nor has acidity been found save in very rare cases. In rare and severe cases petechiæ have appeared on the skin and mucosa.
Lesions of the heart. The implication of the fibrous structures of the pericardium and heart and especially of the valvular structures is a common lesion, and to be dreaded more than all others. In all animals this tendency to cardiac lesion is well marked, but especially in solipeds in which the great demands made on the heart during rapid paces, heavy draught, jumping, etc., severely strain the cardiac mechanism. In dogs there is not only the violent exertion and high blood pressure, but also the great irritability of the nervous mechanism presiding over the heart and the tendency to irregularity and intermissions in the rhythm and palpitations even when the organ is sound. There is every reason to conclude with Trasbot, Megnin, Heu and Laurent that in many cases the heart is primarily attacked, and that this heralds the articular rheumatism. In other cases undoubtedly the cardiac affection is secondary, following the articular attack.
Endocarditis is the most frequent, being found in a large proportion of fatal cases, and thickening of the valves, wart-like exudates and coagula are especially common. The clots may fill nearly the entire ventricular cavity, or at times the auricular, and show a preference for the left side, probably because of the more vigorous systole and the higher blood tension. The clots as a rule are firmly adherent to the diseased valve. Ulceration of the valve is rare in rheumatism. Other parts of the ventricular endocardium may be involved, becoming red, congested, rough or thickened, with adherent blood clots.
Pericarditis is less common though it may exist on either or both the cardiac and visceral folds. It is shown by vascularization, thickening, fibrinous exudate, and serous effusion. Haycock found in a horse a quart of reddish serous exudate with floating fibrous shreds and false membranes. Pus has been found in exceptional cases manifestly indicating a complex infection. Like endocarditis it may precede, follow, or coincide with an articular attack (Leblanc, Cadeac).
Myocarditis is usually seen as a complication and extension of rheumatic inflammation of the visceral pericardium, or of the ventricular endocardium. The muscular tissue may appear parboiled and friable, and shows granular or fatty degeneration.
Pulmonic and pleuritic lesions. Embolism of the lungs and pleura may occur from the transference of clots from the right heart, yet the sequence is much more commonly an articular rheumatism following infective disease of the lungs. Cadeac suggests that the impaired nutrition in pneumonic and pleuritic animals predisposes to the rheumatic arthritis, and again that the microbes of the infectious chest affection, colonizing the joints and other synovial sacs, determine the rheumatism. This last theory has the weakness of identifying influenza and contagious pneumonia with articular rheumatism, and is negatived by the experience that these two pulmonary affections never develop de novo from simple rheumatism. The rheumatism which follows influenza and contagious pneumonia therefore must either be considered as a pseudo-rheumatism, or a rheumatism occurring only concurrently and accidently with the pulmonary affection. Apart from this, pleurisy or even pneumonia occurs as a simple extension from a rheumatic pericarditis.