The differences in the shape of the male and female body are the result of a great many complicated factors and a whole host of minor changes which begin to take place even before birth. They affect not only the primary and secondary sex characteristics, but also the construction of bones, muscle and the various positions in which fat may accumulate. They extend to include the differences in outlook and personality.
We are here interested mainly in the bony structure of the skeleton of the woman rather than in the other distinguishing features that make her different from her male counterpart. It would be comparatively simple to design a mechanism whereby both the legs were jointed immediately to the lower end of the spine, but in order to provide a birth canal the bones are formed in the shape of a circle. The upper parts of the legs are inserted into the lower and outer portions of this circle and the upper and rear portion is attached to the spine, so that weight is transmitted from the legs, round the pelvic girdle, to the spine. The shape of the cavity of the pelvis is important because the baby must pass through it during delivery, and hence it is known as the birth canal. In the female the cavity of the pelvis is round and smooth, whereas in the male it tends to be more heart-shaped and distorted by several bony protuberances.
The pelvic girdle is made up of three bones—the sacrum at the back and two innominate bones at each side. The innominate bones join in front at the symphysis pubis and they join the sacrum at what are known as the sacro-iliac joints. The coccyx is the rudimentary tail at the lower end of the sacrum.
The size of the cavity of the pelvis has to be sufficiently large for the baby’s head to pass unimpeded and at the same time to accommodate the pelvic organs.
The cavity is not a straight tubular structure as so many people imagine, but is short in front and. long at the back, so that once a baby’s head has entered the pelvis it has to turn through an angle of 90° before it is delivered.
The Symphysis Pubis (Pubic Symphysis)
This joint in the front of the pelvic girdle is comparatively small and fairly narrow. It is, however, extremely well supported by ligaments above, below and behind. The joint itself is filled with cartilage and under normal circumstances it moves very little, if at all. Obviously the symphysis pubis, which has to support the front of the pelvic girdle, must be strong enough to withstand the stresses and strains imposed upon it, especially when moving the weight from one leg to the other. In the male the joint is virtually sealed throughout adult life. In the female, however, the joint opens slightly during pregnancy because the ligaments relax, thus enabling the bones to move. During labour the bones of the symphysis may separate by as much as half an inch. This movement can be appreciated if a finger is placed directly over the joint and the weight is moved from one leg to the other.
The sacrum is the curved bone which forms the rear part of the pelvic girdle. It consists of five vertebrae that have been fused together and joins with the spine itself above and the coccyx below. On either side it is attached to the innominate bones by means of the sacro-iliac joints. The size of the brim or inlet of the pelvis depends on the breadth of the sacrum, while the size of the cavity depends on its curvature. The sacrum should be broad, well curved and tilted well backwards in order to form a generous pelvic cavity. If it is unduly narrow then the size of the brim of the pelvis is restricted, or if it is flat the cavity of the pelvis itself is reduced.
The coccyx is the human’s vestigial tail. It is of no real importance and is deflected backwards when the baby’s head passes into the outlet of the pelvis during delivery.
The Sacro-iliac Joints
The long and complicated sacro-iliac joints which are situated between the sacrum and the innominate bones on either side are held firmly together by ligaments and fibrous tissue. The whole weight of the body is transmitted through them since they connect the spine and the legs, and it is for this reason that they do not permit any movement to occur after adolescence, except during pregnancy, when their supporting ligaments and fibrous tissue soften and lengthen to allow the symphysis pubis in the front of the pelvis to open slightly and increase (at least temporarily) the size of the cavity of the pelvis. This movement is comparatively small and takes place only during the latter part of pregnancy. The joints fuse again shortly after delivery.
There are obviously a large number of very strong ligaments sup-porting the joints in the pelvis, because they have to transmit the whole weight of the body both to and from the legs. The ligaments that support the symphysis pubis and the sacro-iliac joints are of the utmost importance but there are also strong ligaments that bridge the spaces on the side wall of the pelvis between the lower part of the sacrum and the innominate bones,
The Birth Canal
The birth canal extends from the inlet or brim of the pelvis to the pelvic outlet. Its direction changes through one complete right angle as it travels through the pelvic cavity, the symphysis pubis being very short and the sacrum long, and it necessarily follows that the baby himself has to turn through a right angle as he passes through the pelvic cavity during labour. If the pelvic brim were at right angles to the mother’s spine, so that the baby could fall directly downwards out of the abdomen, it would mean that the support of the pelvic floor would be very inefficient, whereas the mechanism of having the pelvic brim tilted on the spine ensures that all the contents of the abdomen do not drop straight out as if through a trap door when the baby is delivered.