The TRUE PELVIS, a knowledge of which is so essential to the accoucheur, is divided into tbe brim, the outlet, and the cavity.
The situation of the BRIM is marked by the linea-ilio-pectinea just described, and is of an irregular heart-shaped figure; a line from before backwards, viz. from pubis to sacrum, measures four inches: this is the shortest, and is called the conjugate, or anteroposterior diameter. The lateral, measured from side to side, is five inches. The diagonal, from the acetabulum to the opposite sacra-iliac symphysis, measures five and a quarter, or five and one eighth, inches.
The OUTLET. Its shape, with the soft parts attached, is irregularly quadrangular, the diameters of which are about four inches cither way, the measurements being taken from the symphysis pubis to the os coccygis, and from one tuberosity to the other: but it is to be remembered that the os coccygis yields to the pressure of the child's head during labour, receding a full inch, which will enlarge the diameter of the outlet from before backwards lo this extent, whilst the transverse measurement remains the same. Thus, it is to be observed that the long diameters of the brim and the outlet are opposed to each other; the former being from side to side, the latter from before to behind.
The CAVITY is that part situated between the brim and the outlet, bounded posteriorly by the concavity of the sacrum, anteriorly by the symphysis pubis, and laterally by the tuberosities of the ischia. The depth of the different parts of this cavity should he borne in mind when the situation of the child is examined. Anteriorly, the depth is only two inches; posteriorly, it is from five to six inches; and laterally, it is three inches and a half.
If an articulated skeleton be examined, it will be seen that the pelvis has an oblique hearing upon the trunk; the brim being neither horizontal nor perpendicular, but placed at a very considerable angle. In consequence of this arrangement, the gravid uterus, after it lias risen out of the pelvis, has a tendency to fall forwards, and is then supported by the brim. Where the pelvis is very capacious, the uterus is not supported by this part, but sinks into the cavity, by which many unpleasant symptoms are produced during the latter periods of gestation. Further, it is to be noticed that the shape of the pelvic cavity is described by a curved liue; the axis sf tlic brim being downward and backward, whilst that of the outlet is downward and forward.
When assistance is required in cases of difficulty, this knowledge is of no small importance, as we are enabled to adapt the line of motion to the axis of that particular part of the pelvis through which the child is being brought. This knowledge is of equal avail whether we are delivering by the head or by the feet.
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From Charles Waller
Elements of Practical Midwifery: Or, Companion to the Lying-in Room, 1829
