The =ischium= (Os ischii) forms the posterior part of the ventral wall
or floor of the bony pelvis. It slopes a little downward and inward, but
is practically horizontal in the longitudinal direction. It is
irregularly quadrilateral, and may be described as having two surfaces,
four borders, and four angles.
The =superior= or =pelvic surface= (Facies pelvina) is smooth and
slightly concave from side to side.
The =inferior surface= (Facies externa) is nearly flat, and is in great
part roughened for the attachment of the adductor muscles.
The =anterior border= forms the posterior margin of the obturator
foramen.
The =posterior border= is thick and rough. It slopes obliquely inward
and forward to meet the border of the other side, forming with it the
=ischial arch= (Arcus ischiadicus).
The =internal border= meets the opposite bone at the symphysis.
The =external border= is thick and rounded, but concave in =its length=;
it forms the =lesser sciatic notch=, the lower boundary of the lesser
sciatic foramen.
The =antero-internal angle= or =symphyseal branch= (Ramus symphyseos)
meets the pubis, with which it forms the inner boundary of the obturator
foramen.
The =antero-external angle= or =acetabular branch= joins the other two
bones at the acetabulum, of which it forms more than half. Superiorly it
bears part of the =superior ischiatic spine= (Spina ischiadica), and
internally it is grooved for the obturator vessels.
The =postero-external angle= is a thick three-sided mass, the =tuber
ischii= (Tuber ischiadicum); its lower border is the =inferior ischiatic
spine=, to which the biceps femoris and semitendinosus muscles are
attached.