The two =proximal= or =great sesamoids= (Ossa sesamoidea phalangis
primæ) are situated behind the distal end of the large metacarpal bone,
and are closely attached to the first phalanx by strong ligaments. Each
has the form of a three-sided pyramid. The =anterior= or =articular
surface= conforms to the corresponding part of the distal end of the
large metacarpal bone. The =posterior= or =flexor surface= is flattened
and oblique; in the fresh state it is covered by a layer of cartilage
which also fills the interval between the opposed borders of the two
bones, and forms a smooth groove for the deep flexor tendon. The
=abaxial surface= is concave, and gives attachment to part of the
suspensory ligament; it is separated from the posterior surface by a
rough everted border. The =base= faces downward, and furnishes
attachment to the inferior sesamoidean ligaments. The =apex= is directed
upward and is rounded.
The =third sesamoid= or =navicular= bone (Os sesamoideum phalangis
tertiæ) is shuttle-shaped, and is situated behind the junction of the
second and third phalanges. Its long axis is transverse, and it
possesses two surfaces, two borders, and two extremities. The =articular
surface= (Facies articularis) faces upward and forward; it consists of a
central eminence, flanked by concave areas, and articulates with the
distal end of the second phalanx. The =flexor= or =tendon surface=
(Facies flexoria) is directed downward and backward. It resembles the
articular surface in form, but is more extensive and not so smooth. In
the fresh state it is coated with cartilage and the deep flexor tendon
plays over it. The =proximal border= (Margo liber) is wide and grooved
in its middle, narrower and rounded on either side. The =distal border=
(Margo ligamenti) bears in front a narrow facet for articulation with
the third phalanx. Behind this is a groove, which contains a number of
relatively large foramina, and is bounded behind by a prominent edge.
The =extremities= are blunt-pointed.
[Illustration:
FIG. 59.—THIRD SESAMOID OR NAVICULAR BONE OF HORSE. (After Schmaltz,
Atlas d. Anat. d. Pferdes.)
]
=Development.=—It ossifies from a single center.