Obstetrics : the science and the art . , one is looking downwards into the pelvis, and in Fig. 5upwards through its inferior strait. In Fig. 4, the distance from pro-montory to pubis ought to be at least 4 inches from a to b. The dis-tance across the opening from e toy, should be 4J inches; and the twooblique diameters, c <j, c y,should be five inches Fig. 5, however, the longestline is that from front to rear,and not the transverse or ob-lique ones, so that the longestdiameters of the two straitsdo not coincide, which ren-ders it necessary for the childto make a spiral turn or revo
Obstetrics : the science and the art . , one is looking downwards into the pelvis, and in Fig. 5upwards through its inferior strait. In Fig. 4, the distance from pro-montory to pubis ought to be at least 4 inches from a to b. The dis-tance across the opening from e toy, should be 4J inches; and the twooblique diameters, c <j, c y,should be five inches Fig. 5, however, the longestline is that from front to rear,and not the transverse or ob-lique ones, so that the longestdiameters of the two straitsdo not coincide, which ren-ders it necessary for the childto make a spiral turn or revo-lution as it descends fromabove, to be born—a turn thatis technically called rotation of the foetus. I shall, in another page, speak of the lower strait, asconsisting of a double plane. Most of the bony resistance experienced in labors, is resistancewith at one or the other of these straits, though it is true that we some-times meet with cases in which the excavation itself does offergreat obstruction to the birth of the child. 42 THE PELVIS. I have pointed out the ilio-pectineal or ilio-pubic line, and I wishto say that it is the boundary or margin of the superior strait. Letit be occupied with an imaginary superficies, and call that superficiesthe plane of the superior strait. A line falling perpendicularly uponthis plane just midway between the pubis and the sacrum and in themiddle of the transverse diameter, is the axis of the superior strait;or, to speak more precisely, it is the axis of the plane of the superiorstrait. Now, as such perpendicular line does not correspond with thelong axis of the trunk of the body, but comes out of the trunk at ornear to the umbilicus, and touches the lower end of the sacrum withinthe pelvis, the plane of the strait is an inclined plane, and it is inclinedmore or less according to the posture assumed by the patient or thesubject. In general, the inclination of this plane is about 50°. If theindividual stretches herself upwards, and
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