TENDON OF FLEXOR PERFORATUS— LUXATION 141 



becomes inserted into the depression on the summit of the tuber calcis 

 the two tendons become separated from one another, and the tendon of 

 the perforatus inclines backwards. It then loses its cord-like form and 

 becomes very much expanded, so that it forms a kind of curved sheet, 

 the anterior surface of which is concave and is moulded to the surface of 

 the more posterior of the two convexities at the summit of the tuber after 

 the manner of a cap. On either side a small slip of insertion is detached 

 from the tendon, which becomes attached to the side of the tuber just 

 below the summit. The tendon then becomes much thicker, and runs 

 down the back of the calcis behind the calcaneo-metatarsal ligament. 

 The function of the two lateral slips is to maintain the perforatus tendon 

 on the summit of the tuber. Occasionally one or other of the slips is 

 ruptured, with the result that the tendon becomes displaced. Dis- 

 placement, of course, occurs towards the side opposite to that of the 

 ruptured slip. The tendon of the perforatus passes from the inner 

 side of that of the gastrocnemius to the summit of the calcis. The 

 inner slip is the more frequently ruptured and outward luxation the 

 more common. 



Rupture with subsequent displacement of the tendon occurs as a 

 result of heavy strain upon the part, or it may be due to kicks or other 

 injuries. 



When displacement is due to rupture without any external injury 

 the animal does not show severe lameness. The hock is still extended 

 by the gastrocnemius. But it will be evident that the animal does not 

 possess complete control over the joint, which will be observed to 

 " wobble " from side to side during progression. 



In some cases the tendon may be seen to slip off the summit of the 

 calcis every time the hock is extended, and to resume its position on the 

 summit when the joint is flexed. In other cases where the tendon 

 remains displaced diagnosis is easy, since the displaced tendon can be 

 very readily felt. 



When the rupture is the result of a kick or some other external 



