Although these are the proportions of the brim cavity and outlet of the female pelvis in the skeleton state, their real dimensions during life, when the pelvis is thickly lined with muscular and other structures, are very different. The large masses of the psoas magnus and iliacus internus, besides other muscles of inferior size, contribute to alter materially the relations of the pelvic diameters to each other; hence we find that, so far from being the longest, the transverse diameter is one of the shortest, being little more than the antero-posterior. This holds good, especially during labour, because these muscles being thrown into powerful contraction, their bellies swell, and thus tend still farther to diminish its length. The oblique diameters are, in fact, the longest during life, because not only are the parietes of the pelvis at the brim covered by a very thin layer of soft tissues in these directions; but as the extremities of these diameters, in the cavity and outlet, correspond to free spaces which are merely filled up with soft yielding structure, it follows that their length can be somewhat increased when pressure is applied in these directions; the antero-posterior diameter of the outlet can alone be compared with the oblique diameters in this respect, and then only when the coccyx is forced backwards to its full extent by the pressure of the head. _Pelvis before puberty._ The proportions of the adult female pelvis are no longer what they were during childhood; before the age of puberty they resemble those of the male pelvis, the brim being contracted and more or less triangular, and the antero-posterior diameter equalling or even exceeding the transverse. Indeed, at a still earlier period, it presents many points of resemblance even to the pelvis of animals; as, however, growth and development advance, and the various changes which constitute puberty take place, the transverse diameters of the brim, cavity, and outlet increase at the expense of the antero-posterior, until at length, it has assumed the proper proportions of the adult female pelvis. _Axes._ Of not less importance is it that the obstetrician should be thoroughly acquainted with the direction which the central line or axis of the entrance and outlet of the pelvis takes. The axis of the superior aperture has been considered to form with the horizon an angle varying between 50° and 60°; this was noticed long ago by Dr. Smellie: "when the body of a woman," says this valuable author, "is reclined backwards, or half sitting half lying, the brim of the pelvis is horizontal; and an imaginary straight line, descending from the navel, would pass through the middle of the cavity; but in the last month of pregnancy such a line must take its rise from the middle space between the navel and scrobiculus cordis in order to pass through the same point of the pelvis." (_Treatise of Midwifery_, book i. chap. i. sect. 2.)