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 foraminæ, 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 foraminæ 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 foraminæ. As the name 'laminal' indicates, it is this surface which in the fresh state is covered by the sensitive laminæ. _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 foraminæ; 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. _The Superior or Articular Surface_ consists of two shallow depressions, divided by a slight median ridge. Its posterior part shows a transversely elongated facet for articulation with the navicular bone.