Internus

Internus is a Latin adjective meaning "internal" or "inside." In medical and anatomical terminology, it is used to indicate structures situated within another structure or on the inner side relative to a reference point. It is often paired with externus (external) to denote opposing positions.

In anatomy, internus appears in a range of term names describing internal or inner relations. The obturator internus is a muscle of the hip that originates from the inner surface of the obturator foramen and membrane, passes through the lesser sciatic foramen, and inserts on the greater trochanter of the femur; it assists in lateral rotation and stabilization of the hip. The internal intercostal muscles (intercostales internos) lie between the ribs and aid in forced expiration by depressing the ribs. The internal anal sphincter (sphincter ani internus) is a smooth muscle that helps maintain continence. The internal acoustic meatus (meatus acusticus internus) is a canal in the temporal bone that transmits the facial and vestibulocochlear nerves and the labyrinthine artery.

Other examples include the internal urethral sphincter (sphincter urethrae internus) at the neck of the bladder, involved in urine control. The term is widely used in anatomical nomenclature to label internal or inner relationships and is contrasted with externus for external positions. The usage reflects Latin roots common in medical terminology and functions as a descriptive modifier within compound names rather than as an independent eponym.