Thus it becomes apparent that the structure of the pelvis requires the child’s head, not only to rotate in its passage through the birth canal, but also to describe an arc, since the part of the head which passes down the posterior wall travels farther in a given time than the part which passes under the pubis. This twisting and curving of the birth canal must be appreciated in order to understand the mechanism of labor. In considering the question of pelvimetry, we find that there are both external and internal measurements to be taken, all for the purpose of estimating as accurately as possible the shortest diameter of the inlet through which the baby must pass. (Fig. 5.) According to a common system of mensuration, the first external measurement is the _inter-spinous_, the distance between the anterior-superior spines, those bony points which are uppermost as the patient lies on her back. This distance is normally 26 centimetres. (Fig. 6.) [Illustration: FIG. 5.—Two types of pelvimeters frequently used in taking measurements of the pelvic inlet and outlet. ] The second measurement is the _inter-crestal_, or the distance between the iliac crests, and is normally 28 centimetres. _Baudelocque’s diameter_ is the third measurement and is taken with the patient lying on her side. (Fig. 7.) It is the distance from the top of the symphysis to a depression just below the last lumbar vertebra. This depression is easily located as it also marks the upper angle of a space just above the buttocks, which in normal pelves is quadrilateral. In malformed pelves this quadrangle may be so misshapen as to become almost a triangle with the apex directed either up or down. This dimension is sometimes called the _external conjugate_ and ordinarily measures 21 centimetres. The fourth measurement is the distance between the _great trochanters_, or heads of the femora, and normally is 32 centimetres. All of these measurements, which after all are only approximate, relate to the top of the pelvis and are valuable in that they help in estimating the dimensions of the inlet, which are the important ones, and obviously cannot be measured on a live woman. [Illustration: FIG. 6.—Diagram showing method of measuring distances between iliac crests and spines and the trochanters. ] The inlet has four measurements of obstetrical importance: the _antero-posterior_, or _true conjugate_, which is the distance from the top of the symphysis pubis to the prominence of the sacrum, and is normally 11 centimetres; the _transverse diameter_, which is at right angles to the true conjugate and is the greatest width of the inlet, measuring from a point on one side of the brim to the corresponding point on the other, is normally 13.5 centimetres, and the two diagonal measurements, known respectively as the _right_ and _left oblique diameters_, which are normally 12.75 centimetres.