Interrossei

Interrossei is not a standard term in modern anatomical nomenclature. In most references the muscles and structures between bones are called the interossei (singular interosseus). If encountered, interrossei is typically a variant spelling or translation referring to the same group of interosseous muscles found in the hand and foot, and to the interosseous membrane between forearm bones.

In the hand, the interossei consist of dorsal interossei and palmar interossei. There are four dorsal interossei and three palmar interossei. They originate from adjacent sides of metacarpal bones and insert into the bases of the proximal phalanges and the extensor expansions. Their primary actions are to abduct the fingers (dorsal interossei) and to adduct them (palmar interossei) relative to the middle finger. Both groups assist with flexing the metacarpophalangeal joints and extending the interphalangeal joints. Innervation is via the deep branch of the ulnar nerve (C8–T1), with blood supply from branches of the deep palmar arch and related arteries.

In the foot, interossei are the plantar interossei (three) and dorsal interossei (four). They arise between metatarsals and insert toward the proximal phalanges and extensor expansions. Plantar interossei adduct the toes toward the second toe; dorsal interossei abduct the toes away from the second toe. Together with surrounding musculature, they support toe movements during gait and help stabilize the metatarsophalangeal joints. They are innervated by the lateral plantar nerve, and receive vascular supply from plantar metatarsal arteries.

The term also appears in reference to the interosseous membrane between the radius and ulna, a fibrous sheet that stabilizes the forearm and serves as a site for muscle attachments, distinct from the hand and foot interossei.