Joints are formed of bones which fit into each other by unequal surfaces, whose cavities and eminences mutually correspond. As the roughness of the bones would prevent their moving freely without friction, we find an intervening smooth elastic substance called cartilage, which not only obviates this inconvenience, but in a great measure takes off the jar that would otherwise ensue when the horse moves on hard ground. To render the motion still easier, we find the cartilage covered with a fine synovial membrane, secreting a fluid of an albuminous and oily character, which acts in the same capacity as oil to machinery, namely, to prevent friction.
Besides the membranous connection of bones which enter into the formation of joints, there are strong, flexible substances of a fibrous texture called ligaments, which are the chief bond of attachment, and support the lower joints; while others are further strengthened by muscles and tendons. Bursæ mucosæ are small closed sacs which surround the tendons wherever there is friction; they are analogous in structure to the synovial membrane, and secrete a similar fluid.
Synovitis
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.