This, the =metacarpo-phalangeal articulation= (Articulatio
metacarpo-phalangea), is a ginglymus formed by the junction of the
distal end of the large (third) metacarpal bone, the proximal end of the
first phalanx, and the proximal sesamoid bones.
=Articular Surfaces.=—The surface on the large metacarpal bone is
approximately cylindrical in curvature, but is divided into two slightly
unequal parts by a sagittal ridge. This is received into a sort of
socket formed by the first phalanx below and the two sesamoids together
with the intersesamoid ligament behind. The latter is a mass of
fibro-cartilage in which the sesamoid bones are largely embedded. It
extends above the level of the sesamoids, and is grooved to receive the
ridge on the metacarpal bone; its posterior surface forms a smooth
groove for the deep flexor tendon.
[Illustration: