Читать книгу A System of Midwifery - Edward Rigby - Страница 6

In the annexed representations of the superior and inferior aspects of the female pelvis are shown the three diameters of its brim and outlet; those of the former in fig. 1., and those of the latter in fig. 2. The same letters of reference are used in each figure to indicate the several diameters; thus a p refers to the antero-posterior, t t to the transverse, o o to the right oblique, and o′ o′ to the left oblique diameters.

In fig. 2. the coccyx is represented in situ.

In the cavity these dimensions vary more or less. The straight diameter, measured from the centre of the hollow of the sacrum to that of the symphysis pubis, is 4·8 inches; the transverse, from the point corresponding to the lower margin of the acetabulum on one side to that of the other, 4·3; and the oblique, drawn from the centre of the free space formed by the sacro-ischiatic notch and ligaments on one side of the foramen ovale of the other, 5·2.

At the inferior aperture or outlet the alteration is still more remarkable. The straight diameter, from the point of the coccyx to the lower edge of the symphysis pubis, measures only 3·8 inches; but from the mobility of the coccyx enabling it to be pushed back during labour to the extent of a whole inch, it is capable of being extended to 4·8 inches. The transverse diameter from one tuberosity of the ischium to the other, measures 4·3 inches: and the oblique, from the middle of the lower edge of the sacro-sciatic ligament of one side, to the point of union between the ischium and descending ramus of the pubes on the other 4·8 inches.

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.)

Inclination of the pelvis. The angle which the axis of the superior aperture of the pelvis forms with the horizon, when a woman is in the upright posture, necessarily marks what has been called the inclination of her pelvis, and varies, of course, in proportion to the angle which the above mentioned axis forms. In a tall woman of slender figure, where the different curves of the spinal columns are slight, the inclination of the pelvis is much less than in a short thick set woman, where the spine is much more strongly curved. Where the inclination is slight, the hollow of the sacrum is generally small, and the vulva directed more forwards; where, on the other hand, the pelvis is much inclined, the hollow of the sacrum is generally observed to be deep, and the vulva directed more or less backwards. The axis of the lower aperture or outlet appears to depend, in great measure, on the curve which the lower part of the sacrum takes downwards and forwards; but, as a general rule, we think it will be found to form, more or less, a right angle with the axis of the brim. The greater the angle which the axis of the brim forms with the horizon, the less will be that which the axis of the outlet forms, and vice versâ; or, in other words, the angle with the horizon which the axis of the one forms is inversely to that of the other.

The consideration of the various deviations, as to size and form, from the natural proportions which the female pelvis occasionally presents, belongs, more strictly speaking, to that species of faulty labour which arises from these conditions. We, therefore, refer to the fourth species of dystocia, viz. Dystocia Pelvica, where the different pelvic anormalities are described.