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 foraminae, 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 coronae, 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 coronae.  The articulating surface of the os coronae 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 coronae, 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.

[Illustration:  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.]

[Illustration:  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 of).]

[Illustration:  FIG. 8.—­LIGAMENTS OF THE FIRST AND SECOND INTERPHALANGEAL ARTICULATIONS (VIEWED FROM THE SIDE). (AFTER DOLLAR AND WHEATLEY.) 1, Outermost slip from the glenoidal fibro-cartilage; 2, lateral ligament of the first interphalangeal articulation; 3, prolongations of the lateral ligament of the first interphalangeal articulation attached to the end of the navicular bone to form the postero-lateral ligament of the pedal joint; 4, end of the navicular bone; 5, antero-lateral ligament of the pedal joint.]

The Lateral Ligaments.—­These are large and thick, an outer and an inner, running obliquely from above downwards and backwards.  Each is inserted superiorly into the lateral tubercle of the lower end of the first phalanx, and inferiorly to the side of the second phalanx, their most inferior fibres becoming finally fixed to the extremities of the navicular bone, where they form the postero-lateral ligaments of the pedal articulation.  In front of the joint the extensor pedis plays the part of an additional ligament.