the Ankle or Hock-joint of the Horse. 61 



The external malleolus of the tibia is divided by a deep 

 groove, for the passage of a tendon, into an anterior and poste- 

 rior tubercle ; from the latter of which, and close to the edge 

 of the articulating surface, arises a strong and broad ligament, 

 that is inserted into the os calcis. Under this lies another liga- 

 ment, which, arising from the anterior tubercle, is also inserted 

 into the os calcis. It is to be observed, that the origin of the 

 latter is anterior to that of the former, but its insertion poste- 

 rior, so that these lateral ligaments cross each other in the form 

 of an X. The external articulating protuberance of the astra- 

 galus on which the tibia revolves, has, as has been already 

 stated, a nearly circular outline, and the attachments of the li- 

 gaments just described, are at points on the outside of the os 

 calcis, which would lie nearly in the circumference of that cir- 

 cle, were it continued from the articulating surface; so that 

 each of these ligaments has one of its extremities fixed in a cer- 

 tain point of the circumference, while its opposite extremity re- 

 volves during the motion of the joint, nearly in the circumfe- 

 rence of the same circle. This observation applies likewise to 

 the two lateral ligaments on the inner side of the joint, which 

 have nearly the same relation to each other, and to the general 

 contour of the joint, as that just described ; so it is obvious, that 

 during the rotation of the joint, as the origins of these liga- 

 ments move along the same circumference in which their attach- 

 ments are fixed, the ligaments will be most stretched when they 

 correspond to diameters of that circle. 



. Now it is so arranged that this happens at the same time for 

 all, and consequently the ligaments on each side correspond not 

 merely as to direction, but as to the point of time they become 

 most stretched, which is nearly at the moment that the joint has 

 no tendency to move either way, and at that moment, it is to 

 be observed, that although the ligaments are most tense, and of 

 course react on their points of attachment with greatest force, 

 yet this produces no motion, as the force is exerted in a direc- 

 tion perpendicular to the circumference ; but as soon as the tibia 

 is moved beyond this point of inaction for the ligaments, the 

 latter, no longer representing diameters, by their contractile 

 force evidently tend to accelerate the motion ; and as they all act 

 in the same direction, and are assisted by the shape of the arii- 



