The study of obstetrics is an investigation of the passage, the passenger, and the driving powers of labor, as well as of the various complications and anomalies that may attend the process of reproduction. The passage is composed of a bony canal, called the pelvis, and the soft tissues which line and almost close its outlet. =The pelvis= is made up of four bones; the sacrum, the coccyx, and two other large structures of irregular shape, called the hip, or innominate bones. Joined by cartilage and held in place by ligaments, they form a cavity or basin which, in the male is deep, narrow, small and funnel-shaped, while in the female, slighter bones, expanded openings and wider arches make a broad, shallow channel, through which the child is born. The bony pelvis is divided for description into two parts, the upper or false pelvis, and the lower or true pelvis. The upper pelvis is formed by the wings of the innominate bones and has but two functions of importance to child-bearing. It acts as a guide to direct the child into the true passage, and when measured by the pelvimeter, it gives information as to the shape and size of the inlet to the true pelvis. The true pelvis is of most concern to the obstetrician, because anomalies in its size or shape may impede the progress of labor or render it impossible. The pelvis is divided conveniently into three parts: the brim, the outlet, and the cavity. The _brim_, _inlet_, or _upper pelvic strait_, is the boundary line between the false and true pelvis. It is traced from the upper border of the symphysis along the iliopectineal line on both sides to the promontory of the sacrum. The shape and size of this opening varies much in different races and individuals, both normally and through disease; and when pathologically altered, both shape and size may exercise a marked influence on the course of labor. In American women, the outline of the brim is roughly heart-shaped, like an ovoid with an indentation where the promontory of the sacrum impinges upon the opening. [Illustration: Fig. 1.—The normal female pelvis. (Eden.) The lines _ab_ and _cd_ divide the pelvis into the right and left anterior and the right and left posterior quadrants. _ab_ indicates the anteroposterior diameter of the brim, _cd_ shows the transverse diameter while _gh_ and _ef_ represent, respectively, the right and left oblique diameters.]