3. Changes in the Tendon.—­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 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 originating 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, interfering, 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 the bone are accompanied by well-marked lesions on its gliding or postero-inferior surface, and by evidences of an osteoplastic periostitis along its edges.

That an osteoplastic periostitis has been in existence is witnessed by the appearance along the edges of the bone of numerous outgrowths of bone, termed osteophytes (see Fig. 163).

[Illustration:  FIG. 162.—­A FOOT WITH THE SEAT OF NAVICULAR DISEASE EXPOSED. On the anterior surface of the perforans fibres of the tendon are seen to be torn away from their abnormal adhesion with the navicular bone, while others are seen to be still attached thereto.  The surface of the navicular bone itself exhibits small defects in the bony substance, which have been brought about by a rarefactive ostitis. a, The perforans tendon cut through and reflected; b, the sole.]