DEFINITION.—Idiopathic or constitutional inflammation of the synovial membranes. The inflamed membranes never extend beyond a certain size, nor do they burst, nor do they terminate healthily without treatment, but remain in the same condition from year to year. The inflamed condition is most frequently observed at the hock, when it is called Bog-spavin and Thorough-pin. But it is also found affecting the knee and fetlock, and in the latter case is sometimes confounded with Windgalls, which are inflamed bursæ mucosæ. CAUSES.—Rheumatic Fever; exposure to heat and cold; also friction from quick work on a hard road; sprains. SYMPTOMS.—Lameness quickly succeeded by swelling of some joints, not of the surrounding fibrous texture, as in true Rheumatism; the swelling in this disease is in the synovial cavity, and the effusion is at first generally serous and unattended by the fever which ushers in the muscular or fibrous Rheumatism. As the inflammation proceeds, coagulable lymph may be thrown out and the joint be permanently enlarged, or, from adhesions, the horse be left with a stiff joint. GIVE A.A., four times per day at first, then B.B., morning and night. Ulceration of Articular Cartilage The inflammation in the synovial membrane sometimes extends to the cartilage, covering the ends of the tibia or astragalus. In such cases there is a diminution of the synovial secretion, also ulceration and wearing away of the cartilage, and a polishing of the surface of the bone, which has been erroneously called a porcelaineous deposit. We have seen this ulceration of the cartilage, and even caries of the bone, in the navicular more commonly than in the bones of the hock; but not a few cases of occult lameness in the hock may be attributed to it. Give B.B. two or three times per day. Windgalls—Puffs In the region of the joints, and wherever friction is likely to take place, we find the tendons supplied with little sacs (_bursæ mucosæ_) composed of membrane similar to the synovial, and secreting in health an oily fluid from their internal surface, in very small quantities; but when the tendons become strained, or increased action is set up in them from over-exertion, nature comes to the rescue by increasing the bursal secretion, and we then perceive a slight elastic tumor, called _Windgall_ or _Puff_. CAUSES.—Tendinous sprains or over-exertion of any kind, and long continued friction from quick work on hard roads. Low, marshy pastures seem to have a tendency sometimes to produce a dropsical effusion in and around the joints of young horses, very similar to the enlarged bursæ from hard work; but they soon yield to constitutional treatment.