Occurring in its most marked form, there is no gainsaying the fact that pumiced-foot is a sequel of either acute or subacute laminitis.  As we shall see when we come to study that disease, the dropping of the sole is brought about by distinct and easily-understood morbid processes affecting the sensitive structures.  Briefly, these morbid processes in laminitis may be described thus:  The accumulated inflammatory exudate, and in some cases pus, weakens and destroys the union between the sensitive and insensitive laminae.  This separation, for reasons afterwards to be explained, is greatest in the region of the toe.  The os pedis, loosened from its intimate attachment with the horny box, is dropped upon the sole, and the sole, unable to bear the weight, commences to bulge below.

The altered character of the horn is accounted for by the inflammatory changes in the sensitive laminae and the papillae of the keratogenous membrane generally, for it follows as a matter of course that these tissues, themselves in a diseased condition, must naturally produce a horn of a greatly altered and inferior quality.

When following the subacute form of laminitis, the changes characterizing pumiced-foot are slow in making their appearance.  The animal at first goes short, and the lameness thus indicated gradually becomes more severe, until the animal is no longer able to work.  The feet become hot and dry, the hoof loses its circular form, and the growth of horn at the heels becomes excessive.  At this stage the appearance of bulging at the sole begins to make itself seen.  Later, the outer surface of the wall becomes ‘ringed’ or ‘ribbed,’ the rings being somewhat closely approximated in the region of the toe, and the distance between them gradually widening towards the heels.  The wall too, especially in the region of the toe, instead of running in a straight line from the coronary margin to the shoe, becomes concave.  It is this change, together with the appearance of the rings, that indicates the loosening of the attachment of the os pedis to the wall, and its afterwards backward and downward direction (see Fig. 124).

[Illustration:  FIG. 81.—­HOOF WITH THE RIBS OR RINGS CAUSED BY CHRONIC LAMINITIS.]

As a sequel of acute laminitis, these changes make their appearance with more or less suddenness, and are generally complicated in that they owe their occurrence to the formation of pus within the horny box.

Treatment.—­Pumiced-foot is always a serious condition.  The animal is useless for work upon hard roads or town pavings, and is of only limited utility for slow work upon soft lands.  The more serious form, that following acute laminitis, and complicated by the presence of pus, we may regard as beyond hope of treatment.