610 A MANUAL OF VETERINARY PHYSIOLOGY 



slip of tendon from the abdominal muscles should require to 

 be inserted into the articulating head of the femur. During 

 inward rotation of the thigh — as, for example, when the foot is 

 on the ground and the body is passing over it — the pubio-femoral 

 ligament is rendered tense (Sisson). When the ligament is 

 long, the stifle turns outwards, the points of the hock turn in, 

 and the condition known as ' cow-hocks ' is produced. It will be 

 observed that the production of cow-hocks has nothing to do 

 with any part but the hip-joint. The insertion of the round 

 ligament into the inner side of the head of the femur limits the 

 outward movement of the limb, and is generally considered 

 to explain why a horse so seldom ' cow kicks.' 



The Stifle-joint.— This is the largest joint in the body, and 

 is built on quite different lines to any other. A large convex 

 surface on the femur is placed resting on a flat surface on the 

 tibia, with absolutely no bony cavity in which to rest or play. 

 Shallow cavities are furnished by two plates of cartilage, which 

 receive the condyles of the femur, and on which the latter rest 

 quite superficially. It will be observed that, with this exception, 

 no preparation exists on the tibia for the formation of this immense 

 joint. Nothing could be more unlike in appearance than the 

 in-contact ends of femur and tibia. The discs of cartilage, 

 which take the place of bony cups, no doubt allow of con- 

 siderable flexion and freedom of movement, but as a joint it 

 is unreliable, and consequently the two bones are held together 

 by two crucial ligaments running in opposite directions. These 

 ligaments are somewhat twisted on each other, and when the 

 joint is flexed the posterior ligament is rendered taut. Ex- 

 tension and flexion are the two principal movements of the 

 stifle ; during each of these the discs of cartilage on the tibia slide 

 forward or backward on the bone in conformity with the move- 

 ment of the femur, the movement of the external condyle and its 

 cartilaginous cup being greater than that of the internal. The 

 above is not the only joint formed by the stifle* In front is the 

 patella, playing over two large convex surfaces, the trochleae 

 of the femur. The patella is a floating bone the same as the 

 sesamoids ; it appears to move up and down, but as it is attached 

 below, this is impossible. It is the trochleae of the femur which 

 move, the large inner one doing the bulk of the work. The 

 muscles inserted into the patella and the muscles which flex 

 the hip-joint are direct antagonists. It is impossible for the hip 

 or even so remote a joint as the hock, to flex so long as the 

 patella muscles are maintained in a condition of contraction. 

 When they relax, flexion of the hip-joint is permitted by means 

 of the muscles already considered (p. 596) ; the tibia is drawn up 

 by the biceps femoris and semitendinosus (Figs. 187, 189), the 



