122 THE SURGICAL ANATOMY OF THE HORSE 



angle formed at the tibio-tarsal ioint, by imaginary lines drawn along 

 the longitudinal axes of the tibia and large metatarsal bone, is from 

 155° to 165° depending upon the conformation of the animal. The 

 long axis of the large metatarsal bone is disposed almost vertically, whilst 

 that of the tibia has a forward inclination, to the extent of forming an 

 angle with the vertical of 15° to 25° at the hock. When the joint is 

 flexed the distal end of the limb is carried forwards. This is brought 

 about by the combined action of the flexor metatarsi and extensor 

 pedis muscles, and this movement is so free that the grooves on the 

 tibia pass along the whole length ot the ridges on the astragalus. 

 When thus carried forwards the metatarsus does not move in a vertical 

 plane, but in a plane which runs obliquely downwards and outwards 

 owing to the conformation of the articular surfaces. Extension cannot 

 be carried out to a degree which will bring the long axes of the tibia 

 and the large metatarsal bone into a straight line with one another. 

 This is due to the disposition of the flexor metatarsi muscle, which 

 appears normally too short, and not to the restraining influence of the 

 lateral ligaments as maintained by some writers. This is conclusively 

 proved in cases of rupture of the flexor metatarsi when the joint becomes 

 straightened and the angle almost completely obliterated. 



In the remaining tarsal articulations movement is very restricted, 

 and consists merely of a slight gliding of the articular surfaces on 

 one another. There is very little interference with the action of the 

 joint as a whole in cases where these articulations are obliterated owing 

 to anchylosis of the small bones, provided the process of inflammation 

 has subsided. 



