A PERFECT LUBRICATING SYSTEM 83 



margin of its lower jointed end becomes jammed against 

 the ankle-bone ; the same thing happens when it is bent 

 too far backwards. In life, however, the muscles and 

 tendons passing from the leg to the foot arrest the move- 

 ments before jamming is possible. We must notice, too, 

 another feature of the ankle-joint : when the foot is firm 

 on the ground it is the tibial spoke which rotates ; but if 

 the foot is free and the spoke fixed, then it is the astragalus 

 — the axle of the foot-lever — which rotates. In either 



Lubrical'inq filn 



©(HJ 



Y ' ?M(a 



Middle Scf 



l>owcr &et 



'of Cartilage 



Corpuscles 



■ --Bone corpuscle 



Fig. 24. 



Fig. 23. — A vertical section of the ankle-joint, exposing the lower end of 

 the tibia and astragalus. 



Fig. 24. — A magnification of the part of the ankle-joint cartilage indicated 

 by the line A shown in fig. 23. 



case the lower end of the tibia rubs or glides upon the 

 upper surface of the astragalus. When the whole weight 

 of the body is poised on one leg, as it is with every step 

 we take, it is clear that the lower end of the tibia must press 

 heavily on the upper surface of the astragalus, and unless 

 the joint is provided with a perfect mechanism for pre- 

 venting friction, it must happen that the opposed surfaces 

 will rub each other away until both tibia and astragalus 

 are entirely consumed. 



The mechanism is a simple one. The rubbing surfaces 



