ischii

Ischii is the Latin genitive singular form of os ischii, the ischium, one of the three bones that fuse to form the hip bone (coxal bone). In English anatomy, the ischium refers to the posterior portion of the hip bone that contributes to the acetabulum and to the lower pelvis. The ischium consists of a body and two rami: a superior ramus that connects with the ilium and pubis near the acetabulum, and an inferior ramus that joins the pubis to form the ischiopubic ramus and contribute to the obturator foramen.

Prominent landmarks include the ischial spine, the ischial tuberosity, and the rough, posteroinferior area where sitting presses weight. The greater sciatic notch lies above the spine, and the lesser sciatic notch lies below it; these notches, with surrounding bones, form passages for nerves and vessels. The acetabular notch marks the inferior boundary of the acetabulum and is bridged by the transverse acetabular ligament.

Developmentally, the ischium ossifies from a primary center in the body and secondary centers in the ischial tuberosity and acetabular region, with fusion to complete the hip bone in late adolescence. The tuberosity serves as the major attachment site for the hamstring muscles, while the ischiopubic ramus provides attachments for several surrounding muscles and ligaments.

Clinically, ischial injuries include avulsion fractures of the ischial tuberosity in adolescents and ischial bursitis from prolonged sitting. In Latin anatomical terms, ischii appears as the genitive singular form in phrases such as ramus ischiī, meaning “of the ischium.”