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.
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.
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.
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. The Anterior Border possesses above a small transversely elongated facet for articulation with the os pedis, and below a more extensive grooved portion, perforated by numerous foraminæ, affording attachment to the interosseous ligaments of the articulation. The Posterior Border, thick in the middle, but thinner towards the extremities, is roughened for ligamentous attachment. Development.—The bone ossifies from a single centre.
B. THE LIGAMENTS.
THE ARTICULATION OF THE FIRST WITH THE SECOND PHALANX, OR THE PASTERN JOINT.—Adhering to the limit we have set, this articulation should not receive our attention. As, however, we shall in a later page be concerned with fractures of the os coronæ, which fractures may affect the articulation above mentioned, a brief note of its formation will not be out of place.
It is an imperfect hinge-joint, permitting of extension and flexion, allowing the first phalanx to pivot on the second, and admitting of the performance of slight lateral movements. It is formed by the opposing of the inferior surface of the os suffraginis with the superior surface of the os coronæ. The articulating surface of the os coronæ is supplemented by the addition behind of a thick piece of fibro-cartilage (the glenoid) attached inferiorly to the posterior edge of the upper articulatory surface of the os coronæ, and superiorly by means of three fibrous slips on each side to the os suffraginis. The innermost of these three slips becomes attached to about the middle of the lateral edge of the suffraginis, and the remaining two, beneath the first, attach themselves to nearer the lower end of that bone. The posterior surface of the complementary cartilage forms a gliding surface for the passage of the perforans.
FIG. 6.—THE NAVICULAR BONE (VIEWED FROM BELOW).
1, Inferior surface (smooth for the passage of the flexor perforans); 2, anterior edge of inferior surface; 3, posterior edge of inferior surface.
FIG. 7.—THE NAVICULAR BONE (VIEWED FROM ABOVE, THE BONE TILTED POSTERIORLY TO SHOW ITS ANTERIOR BORDER).
1, Superior articulatory surface; 2, anterior border (grooved portion of); 3, anterior border (articulatory portion