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.
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 Articulations 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.
During flexion, the anterior surface of the foot tends to approach the anterior surface of the leg, the angle formed by these two segments becoming more and more narrowed. The displacement in the opposite direction characterizes extension.
In other quadrupeds, the articulations which bind together the bones of the tarsus possess a little more freedom of movement. The shape of these bones, and particularly the shape of the surfaces of the astragalus, which are in contact with them, allow movements in this region, in the case of the dog and cat, which, without being so extensive as those of the human foot, in the subastragaloid articulation, nevertheless, recall the mobility which we find in the human species at this level—that is to say, rotation, abduction, and adduction of the foot.