When the horny frog is ragged and fissured, and an ill-smelling, dark-colored liquid collects in the lacunæ of the frog, it is affected with thrush. When thrush exists uninterruptedly for several months the perioplic band is irritated and forms rings of periople which assume an irregular course and cross the rings of the middle layer of the wall (Fig. 224). [Illustration: FIG. 224. Hoof with irregular superficial rings resulting from thrush of the frog.] The _causes_: uncleanliness, =too little exercise= in fresh air, excessive paring of the frog, and the use of shoes with calks by which the frog is permanently removed from the ground. The _consequences_ are, besides contraction of the hoof, soreness in travelling, a shortening of the step, and, occasionally, well-marked lameness. _Treatment._—Removal of all greasy horn from the frog, and of the prominent overgrown angles of the buttresses (see page 100), thorough washing of the frog once or twice daily with a 5 per cent. creolin or carbolic solution, =abundant exercise=, and shoes without calks. CHAPTER X. SHOEING MULES, ASSES, AND OXEN. 1. The shoeing of mules and asses is, as in the case of horses, a necessity if these animals are to be used for draft or saddle purposes on hard streets. The structure and characteristics of the hoofs of these animals are quite similar to those of the horse, differing chiefly in the form and thickness of the wall. The mule hoof is long and narrow and round at the toe, the sole is well arched, and the side walls are rather steep (Fig. 225). In the ass the narrowness of hoof is still more pronounced, the wall is relatively =thick=, the frog is particularly well developed in its branches, and therefore the hoof is relatively wide in the region of the quarters. The horn of both mule and ass is tough. The shoes differ from those of the horse in no other respect than that they should be lighter and narrower. Four nail-holes are sufficient for an ass’ shoe, and five to six for a mule’s. On account of the hardness and toughness of the walls, we use nails that are short but strong in the shank; nails with weak shanks are apt to bend in driving. [Illustration: FIG. 225. A mule’s hoof. (Plantar surface).] [Illustration: FIG. 226. A shod ox-claw.] 2. _The shoeing of oxen_ is essentially different from that of horses, because the foot of the ox is cloven (split), the long pastern, short pastern, and hoof-bone are double, so that, instead of one hoof or claw, there are two upon each foot, distinguished as outer and inner. Each claw consists of wall, sole, and bulbs; the frog is absent. The wall is considerably thinner than that of the horse’s hoof, the sole is thin, and the bulbs are low. For these reasons the shoe designed for a claw must be thin, but wide.