AS TO SOUNDNESS. 89 
wedge-shaped navicular bone acts asa 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- 
foroustendon. 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. 
