With respect to solidity, two kinds of horn are distinguished,—namely, =hard= and =soft horn=. The periople, the white line, and the frog are soft horn structures; the middle layer of the wall and the sole are hard or solid horn. The wall, however, is somewhat harder and more tenacious than the sole, for the latter passes off in more or less large flakes (exfoliates) or crumbles away on its lower surface, at least in shod feet, while no such spontaneous shortening occurs in the wall. [Illustration: FIG. 42. Vertical section through middle of a forefoot, the skin and pododerm being in red. (In the figure the direction of both long and short pasterns, _B_ and _D_, is too nearly vertical—too steep). _A_, metacarpal bone (cannon); _B_, os suffraginis (long pastern); _C_, inner sesamoid bone (to render it visible a portion of the intersesamoid ligament was removed); _D_, os coronæ (short pastern); _E_, os pedis (foot-bone); _F_, navicular bone; _a_, extensor tendon; _b_, suspensory ligament of the fetlock; _b′_, superficial inferior sesamoid ligament; _c_, perforatus tendon or flexor of the os coronæ; _c′_, ring passing forward from this tendon and encircling the perforans tendon; _d_, perforans tendon; _e_, capsular ligament of fetlock-joint; _f_, capsular ligament of coronary joint; _g_, _g′_, capsular ligament of pedal joint; _h_, synovial sheath of the perforans tendon; _i_, plantar cushion and fleshy frog; _i′_, bulbs or glomes of plantar cushion; _i″_, indicates the lowest point reached by the plantar cushion, which in the figure is hidden below by the frog-stay of the horny frog; _k_, coronary band (red); _l_, podophyllous tissue (red); _m_, velvety tissue of the sole (red); _n_, velvety tissue of fleshy frog (red); _o_, wall; _p_, sole; _q_, frog; _q′_, the inner half of the frog-stay which reposes in the median lacuna of the fleshy frog; _s_, hair-skin (red).] Soft horn differs from hard horn in that its horn-cells never become hard and horn-like. It is very elastic, absorbs water quickly, and as readily dries out and becomes very hard and brittle and easily fissured and chapped. With respect to _quality_, we distinguish good and bad horn; the former is fine and tenacious (tough), the latter coarse and either soft and crumbling or hard and brittle. If not dried out, all horn is elastic, though soft horn is more elastic than hard. All horn is a _poor conductor of heat_. The relative positions of the various parts of the foot are shown in Fig. 42. Fig. 43 represents the exterior of a well-formed foot. [Illustration: FIG. 43. Right forefoot viewed from the side: _A_, lower end of the cannon; _B_, fetlock-joint; _C_, long pastern; _D_, coronet; _E_, hoof; _F_, heel; _F′_ inner heel; _G_, foot-lock covering the ergot.] CHAPTER II. THE FOOT IN ITS RELATION TO THE ENTIRE LIMB. [Illustration: FIG. 44. Normal (regular) position of fore-limbs.]