ng 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 a