In the horse the ligament of the patella is not single, but consists of three parts, designated, on account of their position, by the respective names of external, internal, and median patellar ligaments. The two former come from the angles on the corresponding borders of the knee-cap; the median springs from the anterior surface and inferior angle of the same bone. They all three pass to their termination on the anterior tubercle of the tibia. The external ligament is the strongest, and the internal ligament the least developed. In the dog, the cat, the pig, and the sheep, the patellar ligament consists of a single band. The articulation is further strengthened on the sides by lateral ligaments--an internal and an external. With regard to the principal movements, these are flexion and extension, to which may be added movements of rotation of limited extent. In flexion, the leg bends on the thigh; its inferior extremity is directed upwards and backwards; the angle which the tibia naturally forms with the femur becomes less obtuse. But it should be understood that one part of this description--that which has relation to the leg--holds good only when the femur is in its normal condition, or in flexion. Indeed, at the close of the movement in which, during a step, the foot is in contact with the ground--that is, at the termination of the resting stage--the inferior extremity of the tibia is directed backwards. But the femur is then in a state of extension, and in regard to this latter the attitude of the leg is unchanged. [Illustration: FIG. 55.--EXTENSION OF THE LEG: RIGHT POSTERIOR LIMB OF THE HORSE, EXTERNAL SURFACE. (AFTER A CHRONOGRAPHIC STUDY BY PROFESSOR MAREY.)] At this moment, notwithstanding the direction, which recalls that which it has at the time of flexion, the leg is not bent on the thigh; on the contrary, it is almost in the line of its continuation (Fig. 55). As we have done in connection with the articulations of the anterior limbs, we borrow this figure from the interesting chronophotographic studies of Professor Marey.[17] [17] E. J. Marey, 'Analysis of the Movements of the Horse by the Chronophotograph' (_La Nature_, June 11, 1898). =The Tibio-tarsal Articulation and of the Bones of the Tarsus.=--In the region which veterinary anatomists call the ham, the articulations of the leg and foot alone call for special study in the case of the horse. The articulations of the bones of the tarsus, and of these with the metatarsus, do not offer any interest with regard to mobility, this being almost wholly absent at that level. The leg and the astragalus, in a general way, are placed in contact by such articular surfaces that the resulting joint, which is a true hinge, permits movements of flexion and extension only. Indeed, as we have indicated above, the tibia is furnished, on the inferior surface, with a crest that fits into the deep groove which is situated on the corresponding surface of the astragalus.