pelvis

pelvis The term ‘pelvis’ comes from the Latin meaning ‘a basin’. It is formed from the two innominate (hip) bones that join to each side of the sacrum at the back, and with each other at the front of the body. The upper part, or false pelvis, houses the lower abdominal viscera, whilst the truly basin-shaped lower part houses and protects the urinary bladder, rectum, and internal genitalia. The innominate (derived from the Latin in nomen, meaning ‘unnamed’) is a large, irregular shaped bone which has been likened to an aeroplane propeller. In the adult, it is formed from the fusion of three separate bones — the ilium above, the ischium below and behind, and the pubis below and in front. These bones fuse in the region of the hip socket (the acetabulum) towards the end of puberty. Typically the female innominate has reached adult form by between 11 and 15 years of age whilst the male takes approximately two years longer to reach maturity. There is only a small temporal window of some 18 months for the female pelvis to transform completely from its juvenile to its final, adult, childbearing form. Such extensive alterations to a skeletal complex arise due to a massive influx of female sex hormones concomitant with puberty. Therefore, those of us with teenage daughters should perhaps try to understand not only the psychological and physiological effects but also the anatomical implications brought about by that raging sea of hormones.

The articulation at the front between the innominate bones is known as the pubic symphysis, and this is a particularly useful area for identifying both the sex and the age at death of skeletal remains, whether they are archaeological or forensic in origin. The articulations between the innominates and the sacrum are at the sacro-iliac joints and, with advancing age, they can synostose (fuse), although this only tends to occur in the elderly male. There appears to be a defensive mechanism in the female that protects the joints of the pelvis in particular and ensures that they remain unfused and able to allow some degree of movement, which can outlast the advantage of this when giving birth. Prior to childbirth, a female hormone called ‘relaxin’ is produced, which serves to loosen both the sacro-iliac joints and the pubic symphysis to permit a few extra millimetres of mobility. This extra room helps the fetal head to squeeze through more easily — although it is still a very tight fit.

Certain clinical conditions, such as rickets, can permanently alter the shape of the pelvis and so have the potential to affect drastically the outcome of childbirth. In periods of history such as the industrial revolution, rickets was rife and as a result feto-maternal mortality was high. It must be remembered that, in the days before anaesthesia and asepsis allowed safe Caesarean section, the only way to expel a fetus was by the natural pathway (per vaginum). If the fetal head could not pass through the pelvis (cephalopelvic disproportion) then labour could last for days and frequently neither the mother nor the baby survived. Some of the more educated women of the time knew that their chances of surviving childbirth were slim at best and so they resorted to many wildly inventive means of preventing pregnancy or, failing that, to ensure abortion of the fetus. It is interesting to note that today, in many of our densely urbanized areas, there is a resurgence of rickets.

The morphology of the pelvis is particularly important in the investigation of human evolution, as it clearly reflects the uniquely bipedal form of locomotion. The junctional position of the pelvis between the trunk above and the legs below ensures that it is intimately involved in the transfer of body weight from the upper body to the ground. This is essential for a bipedal animal, but the female has had to take into account the secondary function of the pelvis, which is, of course, to house a fetus. Not only does the fetus have to be provided with a safe environment for its development, it must have an unobstructed exit that is large enough for its entry into the world. The increased encephalization of the human fetus — the size of its brain and therefore its skull — compared with any other animal has meant that the female pelvis has had to expand even further to ensure safe passage. Therefore the female pelvis is a functional compromise between providing on the one hand the necessary framework for attachment of the muscles that facilitate bipedal locomotion, and on the other a large enough birth canal. The male pelvis does not, of course, have to make any such functional compromise, and it is these fundamental differences between the sexes (sexual dimorphism) that permit the reliable identification of sex from the pelvis.

In the hands of an experienced observer, over 90% of skeletons will be assigned to the correct sex when the innominate bone alone is examined. In fact, not even the entire bone is required for such a degree of reliability. Whilst the front part is the most dimorphic, unfortunately it does not tend to survive burial as successfully as the back part of the bone and so the latter is much more commonly available for the identification of sex. However, a word of caution is needed before relying solely on the innominate for sex determination. Certain racial groups do not exhibit high levels of pelvic sexual dimorphism — for example the Dutch. In this group, the female pelvis is very masculine in appearance and so forensic anthropology achieves a very low level of correct identification. Dutch women are of large body size and so their ancestors have not invested heavily in the remodelling that occurs during puberty. It is interesting to note, however, that the babies born to them are no larger than the European average and hence these mothers enjoy the enviable position of having the lowest level of obstetrical complications in Europe.

Sue M. Black

Bibliography

Aiello, L. and and Dean, D. C. (1990). An introduction to human evolutionary anatomy. Academic Press, London.
Gebbie, D. A. M. (1981). Reproductive anthropology — descent through Woman. J Wiley and Sons Ltd., Chichester.

See also bipedalism; evolution, human; hip; labour; skeleton.