X] THE ORGANS OF LOCOMOTION 119 



We may now consider the joints in the horse's limbs, beginning 

 with the hind leg. 



The Hock Joint. The principal movement of the hock takes 

 place between the tibia and astragalus (a tarsal bone). Here 

 the movement is simple and its range is great, but it is only in 

 galloping and jumping that the angle formed by the tibia and 

 cannon bone is much reduced. The movement between the 

 tarsal bones is small and limited. It is in places of the nature of 

 a rotation, as is shown bj' the presence of grooves on certain of 

 the bones. The greatest amount of pressure comes on the 

 anterior or inner surface of the bones, and it is here that we 

 get the greatest damage in disease. The pressure is removed by 

 flexing the hock which thereb}^ rests the leg. Thus horses kept 

 standing for a considerable time do not usually rest for long on 

 both hind legs equally, but bend the hock of first one leg and 

 then the other in order to escape the effects of the contmuously 

 exerted pressure. 



The Stifle Joint. This isthe largest joint in the horse's body,and 

 it has considerable scope for movement. Owing to the presence 

 of oblique (instead of vertical) ridges on the astragalus a rotation 

 or screw action is produced, not on the hock, but on the stifle. 

 This is turned outwards during flexion of the leg so that it moves 

 clear of the abdominal wall. When the foot is at rest and on the 

 ground the muscles of the stifle (i.e. those passing from the 

 femur to the patella) contract, and thus the leg is maintained in 

 a state of rigidity. 



The Hip Joint. This is a ball and socket joint. Its range of 

 outward movement in the horse is restricted by ligaments in- 

 serted, not into the middle, but into the head of the femur, and 

 on the inner side. This arrangement makes it difficult for a 

 horse to 'cow-kick.' 



The Shoulder Joint. The humerus has a great range of move- 

 ment against the scapula, and thus the joint has very free play. 



The Elbow Joint. The articulating surface is provided with 

 ridges which help to keep the knees in position. 



The Knee {or Wrist) Joint. Here there are three main joints 

 and several lesser ones. The upper of these joints possesses great 

 scope for movements ; the lowest one is much restricted. The 

 'brushing' of the legs together in faulty movement is apparently 

 due partly to the imperfect shape of the articular surfaces between 



