The various kinds of bedding materials might be divided not only into absorbent bedding and drainage bedding, but also into (1) those which exert an injurious influence on the feet of horses stabled on them; and (2) those which do not exhibit that tendency. In the former we may include all kinds of litter which, like peat moss, tan, and sawdust, are bad conductors of heat, and which are liable to become retained ("balled") in the feet in a more or less compact form. When the feet are allowed to continue in this state, even for a few hours in some cases, the soles and frogs, as we may find on removing the offending material, become unduly heated, with the result in time, if the cause be permitted to remain in action, that thrush will become developed and the heels as a rule will become contracted. When we seek for the cause of the heated condition of the frogs and soles from balling, we find that the retention by the feet of a substance which is a good conductor of heat, does not unduly raise their temperature. For instance, "stopping" the feet with clay, which is a good conductor of heat, has as we all know a cooling effect on them. Frequent picking-out of the feet will keep them sound as far as the bedding is concerned.

The ill effects of balling by means of a bad heat conductor are greatly increased, as we might expect, by the litter in question having corrosive or astringent properties. For this reason tan, which contains more or less tannic acid, has a very heating effect on the feet of horses which, like those in many riding schools, are worked on it, or are bedded down on it.

We have previously (p. 47) considered the bad effect which urine has on horses' feet.