DISEASES OF THE JOINTS 431 



3. CJianges in the Tendon. — The effect of these calcareous 

 deposits on the under surface of the hone is to produce a 

 certain amount of roughness. Seeing that with every 

 movement of the foot the perforans tendon is called upon 

 to 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 



