50 THE HORSE IN MOTION. 



the hock; and if the knee and hock joints are both extended it will 

 not change the relations, for the tendon, c, m, and the shaft of the 

 tibia, n, k, being parallel, and the distance from the hock joint, n, to 

 the apex of the hock, c, and that from the centre of motion, /, at 

 the knee to the insertion of the tendon at the femur, m, being equal 

 and parallel, they form a parallelogram, and changes in the angles, 

 as in flexion and extension, will not affect the length of its sides. 

 When the knee or the hock joint is flexed or extended, the other 

 must follow. When the horse is standing, and the knee joint is ex- 

 tended, as well as the hock, the horse rests mechanically upon the 

 tendons, but the knee is extended by the triceps, b, whose tension 

 requires an effect of the will and tires in time, so that we see him 

 when at his ease rest on his hind legs alternately, which he never 

 does with his fore foot, except when one of them is lame. 



The perforans muscle, which is so intimately related to the last 

 has its origin below the knee joint and on the upper and posterior 

 face of the tibia and fibula, below the popliteus (Plate X., e\ and its 

 action is not influenced by the flexions of that joint. Its tendon takes 

 a more direct course to its insertion ; it passes through a groove at the 

 base of the calcaneum, near , on its inner side and as near the joint as 

 possible. Strong ligaments cover the groove where the course of the 

 tendon is changed, to prevent its displacement. It then passes down 

 behind the metatarsal bone and inside the tendon of the perforate 

 On its course it receives the tendon of another small flexor, and fror 

 the posterior surface of the metatarsus an auxiliary tendon or ligament 

 of nearly its own size. In the plate this branch is shown relaxec 

 The tendon, thus reinforced, is of twice the size it was before the 

 union, and passes above the pastern through the ring, r, of the perfc 

 ratus, and is inserted into the bones of the foot. 



This muscle, being entirely independent of the femur and the 

 muscles attached to it, may flex the foot independently, and does so ii 

 propulsion in the last part of the stride, and also in the same contrac- 

 tion aids by its pressure at the back of the hock in extending that 

 joint, thus extending one joint while it flexes another. When the foot 

 rests in the standing position, the auxiliary tendon, t, above mentioned 



