The _first phalanx_ (Phalanx prima)[22] is a long bone, situated between
the large metacarpal bone above and the second phalanx below. It is
directed obliquely downward and forward, forming an angle of 50 to 55
degrees with the horizontal plane in well-formed limbs. It consists of a
shaft and two extremities.
The =shaft= (Corpus) is wider and much thicker above than below, and
presents two surfaces and two borders. The =anterior= or =dorsal
surface= is convex from side to side and smooth. The =posterior= or
=volar surface= is flattened, and bears a triangular rough area, bounded
laterally by ridges which begin at the tuberosities above and converge
below; this area furnishes attachment to the inferior sesamoidean
ligaments. The =borders=, =internal= and =external=, are rounded and
have a rough area or a tubercle on their middle parts.
The =proximal extremity= (Basis) is relatively large. It bears an
articular surface adapted to the distal end of the large metacarpal
bone, consisting of two glenoid cavities separated by a sagittal groove;
the inner cavity is a little larger than the outer one. The posterior
angles are formed by buttress-like tuberosities for ligamentous
attachment. The anterior surface has a slight elevation for the
attachment of the lateral extensor tendon.
The =distal extremity= is smaller, especially in its antero-posterior
diameter. It presents a trochlea for articulation with the second
phalanx, consisting of a shallow central groove and two lateral convex
areas or condyles; the inner area is a little the larger. On either
side, just above the margin of the articular surface, is a depression
surmounted by a tubercle, to both of which the lateral ligament is
attached. Behind the tubercle is a distinct facet to which the
superficial flexor tendon is attached.
=Development.= The first phalanx ossifies from three centers. The distal
end unites with the shaft before birth, the proximal end early in the
first year.
The first phalanx contains a small medullary canal in the middle of
the shaft. It may be remarked that the bone is twisted slightly; when
placed volar surface down on the table, it touches the latter by three
points only, the proximal tuberosities and the internal condyle.