DISEASES OF THE JOINTS 431 
3. Changes in the T'endon.—The effect of these calcareous 
deposits on the under surface of the bone is to produce a 
certain amount of roughness. Seeing that with every 
. Movement of the foot the perforans tendon is called upon 
tv glide over this surface, it is clear that a secondary effect 
must be that of inducing erosion and destruction of the 
tendon. The point at which this usually commences is 
at the bottom of the depression that accommodates the 
ridge on the bone. With erosion of the cartilage and of 
the tendon at points exactly opposite each other, we have 
two surfaces come together that are prone to readily unite, 
and fibrous tissue adhesions often take place between the 
bone and the tendon. In some measure this accounts for 
the torn and ragged appearance of the tendon. Adhesions 
take place, and, under some small strain, are broken down. 
This may happen more than once or twice, and with each 
breaking of the adhesion between the bone and tendon, 
fibres from the latter are lacerated and torn from their 
place (see Fig. 162). 
4. Changes in the Bone.—The changes occurring in the 
bone are essentially those of a rarefactive ostitis. These 
changes are described by many writers, and, whether ori- 
ginating primarily in the bone or not, it seems certain that 
extensive changes may have occurred within the bone, with 
- but little or nothing to be noted on its outer surface. It 
would seem that the first change is one of congestion of the 
vessels of the bone’s cancellous tissue. With the cause, 
whatever it may be, in constant operation, the congestion 
persists until a low type of inflammation is set up, inter- 
fering, not only with the flow of synovia in the adjoining 
bursa, but with the nutrition of the bone itself. As the 
disease progresses, there is softening and enlarging of the 
cancellated tissue towards the centre of the bone. The 
cells break up, and absorption takes place. This goes on 
until a large portion of the interior of the bone is in a state 
of dry necrosis, with, in many cases, but slight signs of 
mischief on the exterior of the bone. 
In other cases, however, the changes in the interior of 
