interossei

Interossei are small intrinsic muscles located between the bones of the hand and the foot. In the hand, they are divided into four dorsal interossei and three palmar interossei. They originate from the adjacent sides of the metacarpal bones and insert into the bases of the proximal phalanges and into the extensor expansions of the fingers.

In the hand, the dorsal interossei are bipennate and insert into the bases of the proximal phalanges of digits 2–4 and the extensor expansions. The palmar interossei are unipennate and attach to the bases of the proximal phalanges of digits 2, 4, and 5 and to the extensor expansions. Their actions include finger abduction and adduction: the dorsal interossei abduct digits away from the middle finger, while the palmar interossei adduct digits toward the middle finger. They also assist the lumbricals in flexing the metacarpophalangeal joints and extending the interphalangeal joints.

Nerve supply for the hand interossei is via the deep branch of the ulnar nerve (C8–T1).

In the foot, interossei consist of four dorsal interossei and three plantar interossei, located between the metatarsals. They arise from the sides of adjacent metatarsals and insert on the bases of proximal phalanges and on the extensor expansions. The dorsal interossei abduct toes away from the second toe; the plantar interossei adduct toes toward the second toe. They contribute to flexion of the metatarsophalangeal joints and extension of the interphalangeal joints. Innervation is by the lateral plantar nerve, a branch of the tibial nerve.