The severity of the lameness will vary with the rapidity of the gait, and with the character of the road upon which the animal is made to travel.  For instance, many animals in which the lameness is imperceptible at a walk become ‘dead’ lame at a fast trot.  It is sufficiently explained when one remembers the greater movements of expansion and contraction of the posterior parts of the wall brought about by the increase in the rate of progression.  The same animal, too, will go distinctly more lame upon a hard than upon a soft surface.

In like manner the lameness from toe-crack also varies in degree with the rate of progression and the character of the travelling, though not to such a noticeable extent as in the lameness from quarter-crack.  A greater variation may in this case be brought about by moving the animal on ascending and descending ground.  Descending an incline, with a more than ordinary share of the body-weight thus thrown upon the heels, the lameness is most marked.  The reason would appear to be that the greater expansion of the wall of the heels thus brought about leads to a proportionate contraction of the wall at the toe, especially at the edges of the crack, thus causing undue pressure upon the exact spot of the wound in the sensitive structures.  Ascending—­the weight in this case transferred from the posterior to the anterior portion of the foot—­the expansion of the heels becomes a contraction, with a corresponding lessening of the contraction at the toe and a distinct decrease in the lameness.

In the case of a deep but recent crack there is always more or less haemorrhage.  This favours risk of infection of the lesion with pus-forming organisms, and so leads to a more or less pronounced lameness, a degree of swelling, heat and tenderness in the coronet above, and a certain amount of surgical fever.

The acute symptoms subdued, but the fissure still remaining, gives us the crack we have classified as ‘old.’  This may in every case be distinguished from a more recent lesion by the amount of thickening of the overhanging coronet, and the presence of an increased quantity of sub-coronary horn in the region immediately about the crack.  The previous inflammatory changes in the adjoining sensitive structures have here led to an increased secretion of horn, and a greater or less deposition of inflammatory connective tissue in the wounded coronary cushion.

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.