The =internal lateral ligament= (Ligamentum collaterale tibiale) is attached above to the prominent internal epicondyle of the femur, and below to a rough area below the margin of the internal condyle of the tibia. The =external lateral ligament= (Ligamentum collaterale fibulare) is somewhat thicker; it arises from the upper depression on the external epicondyle, and ends on the head of the fibula. It covers the tendon of the origin of the popliteus muscle, and a bursa is interposed between the lower part of the ligament and the margin of the external condyle of the tibia. The =crucial ligaments= are two strong rounded bands situated mainly in the intercondyloid fossa of the femur, between the two synovial sacs. They cross each other somewhat in the form of an X, and are named according to their tibial attachments. The =anterior crucial ligament= (Ligamentum cruciatum anterius) arises in the central fossa on the tibial spine, extends upward and backward, and ends on the upper part of the inner surface of the external condyle of the femur. The =posterior crucial ligament= (Ligamentum cruciatum posterius) is internal to the preceding, and is somewhat larger. It is attached to an eminence at the popliteal notch of the tibia, is directed upward and forward, and ends in the anterior part of the intercondyloid fossa of the femur. [Illustration: FIG. 161.—RIGHT STIFLE JOINT OF HORSE; SAGITTAL SECTION PASSING THROUGH OUTER PART OF INNER RIDGE OF TROCHLEA AND INTERCONDYLOID FOSSA. _1_, Anterior crucial ligament; _2_, posterior crucial ligament; _3_, posterior cornu of internal semilunar cartilage; _4_, femoral ligament of external semilunar cartilage; _5_, articular artery; _6_, part of vastus internus. ] It may be added that these ligaments do not lie in a sagittal plane, but are somewhat twisted across each other; outward rotation of the leg untwists and slackens them. =Movements.=—The principal movements of the stifle joint as a whole are flexion and extension. In the ordinary standing position the articular angle (behind) is about 140° to 150°. Flexion is limited only by contact of the leg with the thigh if the hock is also flexed. Extension is incomplete, _i. e._, the femur and tibia cannot be brought into the same straight line. Rotation is limited, and is freest during semiflexion. The patella glides on the femoral trochlea upward in extension, downward in flexion.