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.] The interosseous and postero-lateral ligaments of the articulation often participate in the inflammatory changes, and in many cases become completely ossified. The true articulatory surface of the bone, that articulating with the os pedis and with the os coronæ, is never affected. _Causes_.--In enumerating the causes of navicular disease, we shall follow the example of Colonel Smith and classify them under certain headings--namely, (1) _Hereditary Predisposition_; (2) _Compression_; (3) _Concussion_; (4) _A Weak Navicular Bone_; (5) _A Defective or Irregular Blood-supply to the Bone_; and (6) _Senile Decay_. [Illustration: FIG. 163.--THE NAVICULAR BONE FROM A CASE OF LONG-STANDING NAVICULAR DISEASE. The erosion of the cartilage on its central ridge is most marked, and the porous appearance of the bone thus uncovered points to the existence within it of a rarefactive ostitis. Along its edges large osteophytic outgrowths speak of the effects of an osteoplastic periostitis.]