The pelvis is an essential part of the skeleton which acts as a junction between the abdomen and the legs. It consists of eight key structures: the sacrum, ilium, ischium, pubic bone, pubic symphysis, obturator foramen, acetabulum and coccyx, which are laid out symmetrically. Together these make up the bony pelvis.
The role of the pelvis is threefold. Firstly, it serves as a connecting mechanism between the torso and the legs. Secondly, it is a vital support and balance structure for the upper body. And thirdly it provides a protective, containing cradle for the intestines, bladder and internal sex organs.
The pelvis – technically referred to as the pelvic girdle in this context – consists of a pair of hipbones connected to the base of the vertebral column. Each hipbone is formed from the fusing of three smaller bones (the ilium, ischium and pubic bone) that, when combined, link the base of the spine (the sacrum) to the lower limbs via the acetabula – the cup-shaped cavities into which the femurs fit via ball-and-socket joints.
The pelvic girdle, when fully formed, resembles a roughly cylindrical basin, or the pelvic cavity, perforated on the underside with two main sets of holes. These are the aforementioned acetabula, as well as the obturator foramina, the former receiving the thighbones (femurs) and the latter allowing for the passage of nerves and blood vessels between the torso and the lower body.
Interestingly, in infants the pelvis is a far narrower structure than at full maturity, offering little – if any – support. This changes as we grow, with the pelvis broadening and tilting under the influence of increased walking and standing. When it is fully formed, the human pelvic skeleton can comprise more than ten bones, with the number determined by the composition of the individual’s tailbone (coccyx). This structure is composed of three to five successively smaller caudal vertebrae located at the base of the sacrum.
Male vs female pelvis
Differences between the male and female pelvis are numerous and stem, primarily, from the female’s reproductive system. For example, the female pelvic basin is broader than the male equivalent, the pubic symphysis short and the pubic bones connect in a wider angle with each other. In addition, the coccyx has a greater degree of movement and the two acetabula are spaced farther apart. All together these allow a rounded and spacious birth canal – which hasn’t been an evolutionary priority in the male of the species.