[Illustration: FIG. 156.—LEFT STIFLE JOINT OF HORSE, LATERAL VIEW. The capsules are distended and the external patellar ligament is removed. ] =Articular Surfaces.=—The trochlea consists of two slightly oblique ridges, with a wide and deep groove between them. The inner ridge is much the larger of the two, especially at its upper part, which is wide and rounded. The outer ridge is much narrower, and is more regularly curved; its upper part lies about an inch behind a frontal plane tangent to the inner ridge. The articular surface of the patella is much smaller than that of the trochlea. It is completed internally by a supplementary plate of fibro-cartilage (Fibrocartilago patellæ), which curves over the internal surface of the inner lip of the trochlea. A narrow strip of cartilage is found along the outer border also. The articular cartilage on the trochlea completely covers both surfaces of the inner ridge, but only a narrow marginal area on the external surface of the outer ridge. =Joint Capsule.=—This is thin and is very capacious. On the patella it is attached around the margin of the articular surface, but on the femur the line of attachment is at a varying distance from the articular surface. On the inner side it is an inch or more from the articular cartilage; on the outer side and above, about half an inch. It pouches upward under the quadriceps femoris for a distance of two or three inches, a pad of fat separating the capsule from the muscle. Below the patella it is separated from the straight ligaments by a thick pad of fat, but inferiorly it is in contact with the femoro-patellar capsules. The joint cavity is the most extensive in the body. It usually communicates with the inner femoro-tibial joint cavity by a slit-like opening situated at the lowest part of the inner ridge of the trochlea. A similar, but smaller, communication with the outer femoro-patellar capsule is often found at the lowest part of the outer ridge. The inner communication appears to be constant in adult horses, but is liable to be overlooked on account of the fact that it is covered by a valvular fold of the synovial membrane. It is about half an inch wide, and lies under the narrow articular area which connects the trochlea and internal condyle. The outer communication occurs in 18 to 25 per cent. of cases, according to Baum. It is instructive to distend this capsule and thus obtain an idea of its potential capacity and relations ([Fig. 156]). [Illustration: FIG. 157.—STIFLE JOINT OF HORSE, FRONT VIEW, IN EXTENSION. The capsules are removed. _18_, Femur; _20_, patella; _21_, tibia. (After Ellenberger-Baum, Anat. für Künstler.) ]