[Illustration:  FIG. 3.—­SECOND PHALANX OR OS CORONAE (POSTERIOR VIEW). 1, Posterior surface; 2, gliding surface for passage of flexor perforans; 3, lower articulatory surface.]

Cubical in shape, it is flattened from before to behind, and may be described as possessing six surfaces:  An anterior surface, covered with slight imprints; a posterior surface, provided above with a transversely elongated gliding surface for the passage of the flexor perforans; two lateral surfaces, each rough and perforated by foraminae, and each bearing on its lower portion a thumb-like imprint for ligamentous attachment, and for the insertion of the bifid extremity of the perforatus tendon; a superior surface, bearing two shallow articular cavities, separated by an antero-posterior ridge, for the accommodation of the lower articulating surface of the first phalanx; an inferior surface, also articulatory, which in shape is obverse to the superior, bearing two unequal condyles, separated by an ill-defined antero-posterior groove, which surface articulates with the os pedis and the navicular bone.

Development.—­The bone usually ossifies from one centre, but often there is a complementary nucleus for the upper surface.

THE THIRD PHALANX, OS PEDIS, OR COFFIN BONE.—­This also belongs to the class of short bones.  It forms the termination of the digit, and, with the navicular bone, is included entirely within the hoof.  For our examination it offers three surfaces, two lateral angles, and three edges.

The Anterior or Laminal Surface, following closely in contour the wall of the hoof, is markedly convex from side to side, nearly straight from above to below, and closely dotted with foraminae of varying sizes.  On each side of this surface is to be seen a distinct groove, the preplantar groove, or preplantar fissure, which, commencing behind, between the basilar and retrossal processes, runs horizontally forwards from the angles or wings of the bone, and terminates anteriorly in one of the larger foraminae.  As the name ‘laminal’ indicates, it is this surface which in the fresh state is covered by the sensitive laminae.

The Inferior or Plantar Surface, hollowed in the form of a low arch, presents for our inspection two regions, an anterior and a posterior, divided by a well-marked line, the Semilunar Crest, which extends forward in the shape of a semicircle.  The anterior region, as is the laminal surface, is covered with foraminae; in this case more minute.  In the recent state it is covered by the sensitive sole.  The posterior region, lying immediately behind the semilunar crest, shows on each side of a median process a large foramen, the Plantar Foramen.  From this foramen runs the Plantar Groove, a channel, bounded above by the superior edge, and below by the semilunar crest of the bone, which conducts the plantar arteries into the Semilunar Sinus, a well-marked cavity in the interior of the bone.