Functional anatomy of the finger

Dorsal "hood" and extensor expansion of the third digit. Lateral (A) and posterior (B) views of the dorsal "hood" and extensor expansion. Observe the interosseous muscles inserted in part into the base of the proximal phalanx and in part into the extensor expansion and the lumbrical muscle, which inserts into the radial side of the extensor expansion. (C-D) Lateral views of the extensor expansion in extension. The extensor expansion is a delicate fibrous band that runs from the proximal phalanx and fibrous digital sheath obliquely across the middle phalanx and two interphalangeal joints to join the dorsal "hood" to the distal phalanx. (E) Lateral view of the finger in flexion. On flexing the distal interphalangeal joint (C), the extensor expansion (particularly its lateral bands) becomes taut, pulling the proximal joint into flexion (E). Similarly, on flexing the metacarpophalangeal joint, the proximal and distal joints are pulled by the extensor expansions (lateral bands) into nearly complete extension.