(d) Their Duration.—­Recent when newly formed; old when of long standing.

(e) Their Starting-point.—­This last distinction we make ourselves, and, referring to cracks of the wall, term them high when commencing from the coronary margin, low when starting from the bearing surface.

Causes.—­We have already classified sand-crack as a disease arising from faulty conformation.  Thus, in just so far as a predisposing build of body may be handed down from parent to offspring, we may regard sand-crack as hereditary.  If we do so, however, we must afterwards make up our minds to sharply distinguish between the sand-crack plainly brought about by accidental cause, and that occurring as a result of hereditary evil conformation.

With regard to the latter, we need hardly say that feet with abnormally brittle horn are extremely liable.  But with this, as with many other affections of the feet, we shall find it necessary to consider several causes acting in cooperation.  In this case, for instance, given the brittle horn, it becomes necessary to further look for exciting causes of its fracture.

We will take conformation first.  In the animal with turned-out toes a more than fair share of the body-weight is imposed on the horn of the inner quarter.  Here, then, three causes exert their influence together:  The horn is brittle; the wall of the inner quarter is thinner than that of the outer; additional weight is imposed upon it.  Fracture results.

Take, again, the vice of contracted heels.  Here, in the first place, we have a variety of causes tending to bring about the contraction.  With the contraction, and its consequent pressure upon the sensitive structures in the region of the quarters and the frog, has arisen a low type of inflammation.  The horn of the part has become dry and brittle.  The exciting cause of its fracture is found in an excessive day’s work upon a hard, dry road, with, perhaps, a suddenly-imposed improper distribution of weight, due to treading upon a loose stone, or a succession of such evil transfers of weight due to travelling upon a road that is rough in its whole extent.

In their turn, too, such defects of the feet as we have mentioned in the last chapter—­as, for example, the foot with the pumiced horn, the foot with abnormally upright heels, or that which is upright on one side only, or crooked—­each offers a condition which is predisposing to the formation of a sand-crack.  In each case it wants but the uneven distribution of the body-weight, which, as a matter of fact, some of these conditions themselves give, to bring about a fracture.