This section is from the book "Applied Anatomy: The Construction Of The Human Body", by Gwilym G. Davis. Also available from Amazon: Applied anatomy: The construction of the human body.
The pelvis is composed of the pelvic girdle on each side (innominate bones), and the sacrum and coccyx posteriorly. It serves two purposes. It supports and protects the abdominal and pelvic viscera, and serves as the connection between the trunk and the lower limb. It is divided into two parts - the false pelvis, above the iliopectineal line, and the true pelvis, below the iliopectineal line.
The false pelvis serves to support the abdominal viscera, as its name indicates, like a basin. In man it is large and flaring because his normal position is upright, but in the lower animals, as the quadrupeds, whose normal position is horizontal, it is smaller and less prominent.
The true pelvis contains and protects the pelvic organs and also serves as the connecting link between the trunk above and the extremity below; hence, as it has a double function, it has of necessity a composite structure. In order to contain and protect the pelvic viscera it is made hollow, and in order to support the weight of the body on the legs it is made strong. The pelvic contents are not exposed to injury to the same extent as is the brain; therefore, instead of having a complete covering of bone, like the skull, the bony pelvis is merely a framework comprised solely of those parts essential to strength.
The pelvis supports the trunk in two postures, the standing and sitting. In the former the weight is transmitted through the acetabula, and in the latter to the tuberosities of the ischia.