hion.

Sand-crack of the toe always follows the direction of the horn fibres. That
of the quarter, however, may on occasion run a course that is somewhat
zigzag, first following the direction of the horn fibres for a short
distance, then travelling in a horizontal direction, and finally continuing
its course again in a line with the horn fibres, commonly at a point
posterior to that at which it commenced.

In a quarter-crack that is old, and when contraction of the heels exists
(which in this case it usually does), then will often be found overlapping
of the edges of the crack. The expansion of the wall brought about when the
body-weight is on the heels, cannot, by reason of the break in it, continue
itself anterior to the crack. As a consequence, repeated expansion of the
wall posterior to the crack, with the portions anterior to it in a state
of enforced quiescence, leads in time to the posterior edge of the crack
coming to lie over that of the anterior.

_Complications_.--The first complication likely to arise in a case of
sand-crack is that attending simple laceration of the sensitive structures
in a _deep_ lesion. With the laceration all the phenomena of a repairing
inflammation make their appearance. As a result, there is more or less heat
according to the degree of inflammatory hyperaemia, swelling according to
the amount of inflammatory exudate, and pain according to the amount of
pressure the two foregoing bring to bear on the nerves in the inflamed
area.

A second and more serious complication is the greater inflammation set up
by the introduction into the crack of foreign substances. Small portions of
gravel and flint, both by the irritation set up by their friction and by
the infection they carry in with the dirt surrounding them, are responsible
for the mischief.

When, from direct communication with the blood-stream, due to extensive
haemorrhage, bacteria from the outside gain entrance, this simple
inflammation is further complicated by the formation of pus, or a limited
gangrene of the keratogenous membrane.

In cases of great severity the gangrene of the keratogenous membrane
spreads until the deeper structures are involved. We then get a necrosis
(in the case of toe-crack) of the extensor pedis, and sometimes caries of
the os pedis.

In like manner the necrotic changes occurring under these circumstances may
invade the deeper structures in the region of quarter-crack. As a result of
this, we may