_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 foraminæ. _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.