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.

The Superior Edge, outlining the superior margin of the laminal surface, describes a curve, with the convexity of the curve forward.  In the centre of the curve is a triangular process, the Pyramidal Process, which serves as the point of attachment of the extensor pedis.

The Inferior Edge, the most extensive of the three, separates the laminal from the solar surface.  It is semicircular in shape, sharp, and finely dentated, and is perforated by eight to ten large foraminae.

The Posterior Edge, very slightly concave, divides the small, transversely elongated facet of the superior surface from the posterior region of the inferior surface.

The Lateral Angles of the bone, also termed the Wings, are two projections directed backwards.  Each is divided by a cleft into an upper, the Basilar Process, and a lower, the Retrossal Process.  In old animals the posterior portion of the cleft separating the two processes gradually becomes filled in with bony deposit, thus transforming the cleft into a foramen, which gives passage to the preplantar artery.  We may mention in passing that the lateral angles give attachment to the lateral fibro-cartilages, and that the lateral angles themselves in old horses become increased in size owing to ossification of portions of the adjacent lateral cartilages.

Development.—­The os pedis ossifies from two centres, one of which is for the articular surface; but this epiphysis fuses with the rest of the bone before birth.

[Illustration:  FIG. 4.—­THIRD PHALANX OR OS PEDIS (POSTERO-LATERAL VIEW). 1, Anterior or laminal surface; 2, preplantar foramen; 3, preplantar groove; 4, basilar process of the wing; 5, retrossal process of the wing; 6, foramen caused by the ossifying together posteriorly of the basilar and retrossal processes.]

[Illustration:  FIG. 5.—­THIRD PHALANX OR OS PEDIS (VIEWED FROM BELOW). 1, Plantar surface; 2, plantar foramen and plantar groove; 3, semilunar crest; 4, tendinous surface; 5, retrossal processes of the wings.]

THE NAVICULAR BONE, SHUTTLE BONE, OR SMALL SESAMOID.—­Placed behind the articulating point of the second and third phalanges, this small shuttle-shaped bone assists in the formation of the pedal articulation.  It is elongated transversely, flattened from above to below, and narrow at its extremities.  In it we see two surfaces, and two borders.

The Superior or Articular Surface of the bone, which may easily be recognised by its smoothness, is moulded upon the lower articular surface of the second phalanx, being convex in its middle, and concave on either side.

The Inferior or Tendinous Surface resembles the preceding in form, but is broader and less smooth.  In the recent state it is covered with fibro-cartilage for the passage of the flexor perforans.