Os suffraginis with both sesamoid bones in position, as in [Fig. 3. A], os suffraginis; B, sesamoid bones; a, upper joint-surface of long pastern; b, joint-surface of sesamoid bones; c, roughened surface at upper end; d, roughened surface at lower end, both for attachment of ligaments; e, lower joint-surface.
The short pastern (second phalanx) ([Figs. 5] and [6]) lies under the first phalanx and above the os pedis; it is somewhat cubical in shape. Its upper articular surface ([Fig. 5, a]) presents two glenoid cavities to correspond with the condyles of the first phalanx. The lower articular surface ([Fig. 5, d]) resembles the lower end of the first phalanx. The upper posterior border of this bone is prominent and prolonged transversely ([Fig. 6, a]), to serve as a supporting ledge for the first phalanx, as a point of attachment for the perforatus tendon, and as a gliding surface for the perforans tendon.
Fig. 5.
Short pastern (os coronæ)
viewed in front and in profile:
a, upper joint-surface;
b, anterior surface;
c, lateral surface;
d, lower joint-surface.
Fig. 6.
Short pastern seen from behind:
a, smooth surface over
which the perforans tendon glides;
b, lower joint-surface.
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.