The pelvis serves both to support and protect the reproductive and other pelvic organs. It is a bony ring, sitting between the vertebral column and the hip bone. Four bones combine together to form the bony ring, the two innominate hip bones and the coccyx and sacrum.
Ilium, ischium and pubisCredit: http://commons.wikimedia.org/wiki/File:Pelvis_diagram.png
The innominate bone is actually divided into three parts, namely the ilium, the ischium and the pubis. The ilium is the upper half and the superior border or iliac crest is the one responsible for hip prominence. The ischium is the located below the ilium and the portion at which we sit is actually the lowest part of the ischium called ischial tuberosities. These serve as markers if someone is trying to measure the width of the lower pelvic. Ischial spines, which mark the midpoint of the pelvis, are small projections that extend towards the pelvic cavity. The pubis is located in the front portion of the innominate bone and serves as the junction between the front pelvis and the innominate bone.
The SacrumCredit: http://www.axialif.info/AXI_axialif_eng.html
The sacrum lies between the fifth segment of the lumbar spine and the coccyx which forms the upper front portion of the pelvic ring. When doing measurements of the pelvic, the sacrum is a landmark to remember to be able to get an accurate measurement. The sacral prominence is a projection when the sacrum and the lower lumbar vertebrae touch. The term sacrum is actually used when referring to the four bones of the sacral area, and when referring individually the term sacral vertebrae is used.
The CoccyxCredit: http://www.chiropractic-help.com/Coccyx-pillow.html
Situated inferior the sacrum, the coccyx is comprised of five small bones merged together. In childbearing women, the degree of movement being offered by the joint in between the coccyx and the sacrum allows for the pressing backward of the coccyx, giving more room for the head of the foetus to pass through during child birth.
False and True PelvisCredit: http://en.m.wikipedia.org/wiki/Lesser_pelvis
The pelvis can be further subdivided into the false and true pelvis. This is extremely helpful especially for obstetric purpose. The superior portion of the pelvis is the one referred to as the false pelvis and the inferior portion of the pelvis is the true pelvis. The role of the false pelvis during pregnancy is to give support to the uterus at latter part of pregnancy and serves as the guide for the foetus in locating the true pelvis. The only thing that divides the false and true pelvis is the linea terminalis, an imaginary line. In locating this line, the sacral prominence comes into play. The imaginary boundary is drawn from the sacral prominence to the top portion of the symphysis pubis. The area superior the drawn imaginary line is the false pelvis and the one inferior is the true pelvis.
Inlet, pelvic cavity and outletCredit: http://www.emory.edu/ANATOMY/AnatomyManual/pelvis.html
The entrance to the true pelvis is referred to as the inlet. This is where when a foetus that is born vaginally passes through. It is marked at the level of the innominate line or the linea terminalis. The transverse dimension of the pelvic inlet is much larger compared to the anteroposterior dimension and since the foetus needs to pass through this, important measurements are needed in assessing the pelvis in pregnancy.
The pelvic cavity is a body cavity bounded by the pelvic inlet and the pelvic floor or the outlet. The space conforms the shape of the pelvis, hence a curve space. The curve space also proves to be vital during pregnancy in controlling the speed of the birth, significantly lowering the pressure on the head of the foetus. As the chest of the foetus passes through the cavity, is it being compresses thereby expelling fluids and mucus from the lungs preparing the foetus for better aeration.
These are the major components of the pelvis and if one decides to go deeper and understand more vividly the concepts included in the anatomy of the pelvis, then prepare to be mesmerized. A complex interplay of bones and shapes awaits you and you will marvel at the travel you have gone through during your birth.