AS TO SOUNDNESS. 89 



wedge-shaped navicular bone acts as a mainstay to the coffin 

 joint, and the more the coffin joint needs staying the tighter 

 and firmer is the wedge pressed in the cleft by the per- 

 forous tendon. The bone, having an extremely precarious 

 nutrient supply, and being compressed for hours together, 

 no wonder that disintegrating changes take place in it so 

 often as they do. Where surface meets surface you find 

 the elastic buffer-like provision of articular cartilage 

 opposed to the surfaces of the navicular bone at the 

 coffin joint ; but the articular cartilage over the navicular 

 surface, pressed upon by the perforous tendon, does not 

 meet with the same amount of yielding elastic resistance 

 from the tense hard tendon which presses it forward into 

 the joint; hence the prevalence of degenerative changes 

 in the bone nearest this surface, and involving this surface 

 and the tendon which presses it in perhaps most cases. 



