The brim or inlet has four important diameters to be remembered; important because the hard, round head of the child must pass through them by accommodating its diameters as favorably as possible to those of this opening. These diameters are named respectively the anteroposterior or conjugate diameter, the transverse, and the right and left oblique diameters. The two oblique diameters attain their greatest importance when the pelvis is irregularly distorted, but the others are essential in every case where labor impends. It is to secure an estimate of these latter diameters that the bony prominences are measured. This upper opening lies not horizontally, but in oblique relation to the body in standing position, and the weight of the abdominal viscera rests largely upon the bones and in consequence does not crowd into the inlet unless forced in by corsets or faulty habits.
[Illustration: Fig. 2.—The planes of (_a_) the brim, (_b_) the cavity and (_c_) the outlet. (Eden.)]
Passing through the brim, a cavity is found below it, midway between the inlet and outlet, which is nearly round in shape. This is the “excavation,” or the _true pelvis_. Then comes the _outlet_, bounded in front by the pubic arch and soft parts, and behind by the coccyx pushed back as far as it can go. It is ovoid in shape, but the long axis of this ovoid lies at right angles with the axis of the ovoid inlet.
We find, therefore, a succession of three geometric figures or planes through which the head must pass by means of a spiral motion called rotation. These figures are inclined to one another so markedly in front that a line drawn through the center of each will curve forward at both ends, one end passing out near the umbilicus, the other through the vulva. This is known as the axis of the pelvis or the curve of Carus.
[Illustration: Fig. 3.—Visceral relations. (Redrawn from Gray.)]
Inside the pelvis are the organs of generation with their accessory structures and supporting tissues.
Of first importance are the ovaries, tubes and uterus, together with the vagina. These special structures are the true genital organs. They are bounded in front by the bladder, behind by the rectum, above by the abdominal viscera, and surrounded everywhere by muscular, mucous and fatty tissues, which support them and aid their function.