Apart from the predisposition conferred by conformation, must be remembered the simpler predisposing causes leading to brittleness of the hoof.  We refer to the after-effects of poulticing, the moving from pasture to stable, the emigration from a damp to a dry climate, or the alternate changes from damp to dry in a temperate region.  Each may have a deteriorating influence upon the horn, rendering it liable to the condition we are describing.  Excessive dampness alone, especially when the animal is called upon to labour at the drawing of heavy loads upon a rough road, is not infrequently a cause.  In this case the wet, together with the constant friction of the sharp materials of which the road is made, serves to destroy the varnish-like periople.  The wet gains access to the inner structures of the wall, the agglutination of the horn fibres is weakened, and fissures begin to appear.

Other causes of sand-crack are purely accidental.  An animal at fast work over-reaches.  The secretion of horn at the injured coronet is interfered with, a diminished supply at an isolated spot being the result.  From this point grows down a fissure in the wall.

An injury of the same character may also be sustained in various other ways—­treads from other animals when working in pairs, accidental wounding with the stable-fork, blows of any kind, or a self-inflicted tread with the calkin of an opposite foot—­each with the same result.

So far as causation is concerned, toe-crack stands in a class almost by itself.  It is met with nearly always in a heavy animal in the hind-foot, and is directly attributable to the force exerted in starting a heavy load.

Unskilful shoeing also plays a part in the causation of sand-crack.  Removal of the periople by excessive rasping of the wall is most certainly a predisposing cause.  Cracks, or their starting-points, may also be caused by using too wide a shoe, or by the use of nails too large in the shank.  Also, they may arise from unskilful fitting of the toe-clip, especially in the hind-foot of a heavy animal.  It must be admitted, however, that the part shoeing plays in the causation of sand-crack is not a large one; far more depends upon the state of the horn and the animal’s conformation than upon the exciting cause.

So far, our observations on the causes of sand-crack have referred to that form occurring in the wall.  Sand-crack of the sole or frog we have already said is but seldom met with, and then it is always in connection with some exceptionally deteriorated quality of the horn, as in the case of badly pumiced feet, or occurs as a result of direct injury.  Extensive slit-like cuts in this region, when deep enough to lacerate the keratogenous membrane, are sometimes followed by the growth of a fissure in the horn, and what might almost be termed a permanent sand-crack results.  Such cuts may be occasioned by sharp flints, broken glass, or other sharp objects picked up on the road, or may result from the animal treading on the toe-clip of a partially cast shoe.