The lowest bone of the limb is the =third phalanx= or =os pedis= (Fig. 7). In form it is similar to the hoof. The _anterior or wall-surface_ (_a_) is rough, like pumice stone. Above and in front is the _pyramidal eminence_ to which the tendon of the anterior extensor of the phalanges attaches. Behind, the bone extends backward to form the _inner_ and _outer branches_ (_c, c_) or wings of the os pedis. The _upper_, articular surface (_b_) slopes backward and downward. The _lower_, solar or plantar surface (Fig. 8, _a_) is slightly concave, and presents posteriorly a half-moon-shaped excavation, with a roughened border called the _semilunar crest (c)_, to which the perforans tendon attaches; just above this crest are two small holes (_e_) known as the _plantar foramina_, through which the plantar arteries pass into the bone. The surfaces of wall and sole come together in a sharp edge, which is circular in its course. It is easy to tell whether a pedal bone is from a fore or a hind limb; the os pedis of a hind leg has a steeper and more pointed toe, and a more strongly concaved solar surface than the same bone of a fore-leg. Not only is the outline of the sharp inferior border of the os pedis of a _front foot more rounded at the toe_, but when placed on a flat surface the _toe does not touch_ by reason of being turned slightly upward, much as a shoe designed to give a “rolling motion.” The os pedis of a _hind foot is narrower from side to side_ (pointed), and _does not turn up at the toe_. [Illustration: FIG. 7. Os pedis seen in profile and in front; _a_, anterior face with pyramidal eminence above; _b_, joint-surface; _c_, wings or branches of hoof-bone; _d_, notch which, by the attachment of the lateral cartilage, is converted into a foramen and leads to _e_, the preplantar fissure.] [Illustration: FIG. 8. Lower surface of hoof-bone; _a_, anterior portion covered by the velvety tissue of the sole; _b_, wing of the os pedis; _c_, semilunar crest, to which the perforans tendon attaches; _d_, plantar fissure leading to _e_, plantar foramen.] The right and left hoof-bones are also, as a rule, easily distinguished by variations in the surfaces of wall and sole. The shape of the os pedis corresponds to the form of the horny box or hoof, and therefore a knowledge of this bone is absolutely necessary. The =navicular bone= (os naviculare, nut-bone, Figs. 9 and 10) is an accessory or sesamoid bone to the os pedis. It is a small bone, transversely elongated and situated behind and below the os pedis and between the wings of the latter. It adds to the articular surface of the pedal joint. Its under surface is smooth, and acts as a gliding surface for the perforans tendon, which is quite wide at this point. [Illustration: FIG. 9.] [Illustration: FIG. 10. Fig. 9 represents the upper surface of the navicular bone; Fig. 10 the lower surface of the same: _a_, anterior border; _b_, slight elevation in middle of under surface.]