The false pelvis is formed laterally by the con cave surface of the internal iliac fossm directed upwards, forwards, and inwards ; and poste riorly by the lateral masses of tfie base of the sacrum, directed upwards and forwards. In the middle is also seen, in the articulated pelvis, the anterior surface of the body of the last lurnbar vertebra, filling up, with the pelvi lumbar ligaments, the notch otherwise left be tween the ilia behind. The superior border of the false pelvis is formed by the ilio-lumbar ligaments (N% hich exclude the iliac tuberosities), and the anterior three-fourths of the iliac crest, the most prominent point of which, in the proper position of the pelvis, is the centre of the posterior curve. It is terminated sud denly, in front, by the anterior superior iliac spine, where the ventral notch commences: by the deficiency of osseous structure at this part.
The brim of the pelvis is a heart-shaped opening, formed posteriorly by the body of the first sacral vertebra which overhang,s the cavity of the true pelvis, so as to form a pro jection called the promontory of the sacrunz (i), corresponding to the indentation in the emble matical heart-shape. On each side of this, the rounded arched anterior borders of the lateral masses of the sacral base continue the brim across the sacro-iliac joint, to the thick rounded ridge on the inner surface of the ilium, which is prolonged behind the ilio-pectineal eminence to the horizontal branch of the pubis where the brim becomes identified with the pectineal line. Finally, the brim is completed anteriorly by the shelving border of the body of the pubis, immediately behind the crest, and by the rounded superior part of the pubic symphysis. The part of the brim of the pelvis which is formed by the two portions of the innominate bone is sometimes called the linea ilio-pectinea, or, by some, the linea innonzinata. Sometimes the brim is called the inlet of the true pelvis.
The cavity of the true pelvis is formed laterally by the plane sloping inner surfaces of the lower part of the ilia, opposite the cotyloid cavities, and of the descending branches of the ischia, the latter being termed by obstetricians the planes of the ischia ; in front, by the posterior surfaces of the branches and symphjsis of the pubis, and hy the as cending branches of the ilia ; and behind, by the whole concave anterior surface of the sacrum and coccyx, the former being some times called the hollow of the sacrunz. From • the oblique position of the pelvis, the posterior wall, which is the deepest, also reaches the highest, and the lateral walls the lowest ; the sub-pubic arch cutting out the anterior wall and leaving only the short symph) sis pubis to represent it. The interval between the sa crum and ossa innominata behind, forming the sacro-sciatic notch, is completed and bounded by the sacro-sciatic ligaments, the inner sur faces of which are seen in this view. The inner surface of the coccyx is also seen to have an aspect directed upwards and for wards, and the spines of the ischia to project considerably inwards, so as to present two opposite points, the distance between which may sometimes be of great importance in parturition. This projection is much greater in the male than the female, and will be al luded to in the relative measurements of the pelvis. The cavity of the pelvis contracts uniformly downwards at the sides by reason of the inclination of the innominate bones ; but, from the vertical curvature of the sacrum, the antero-posterior diameter is much greater in the middle than at the superior or inferior outlets, which are hence termed straits. The
presence of the obturator foramina antero laterally, and of the sacro-sciatic foramina postero-laterally, must also be remarked as constituting four opening,s, diagonally op posed to each other, capable, from the yield ing nature of the structures filling them, of enlarging these diameters under sufficient pressure The great projection, forwards, of the coccyx and lower end of the sacrum may be considered as compensated for by the de ficiency of the anterior wall in the sub-pubic arch directly opposite to them, gradually widening downwards as they advance. Both the forward direction of the coccyx, and the width of the pubic arch, are peculiar to the human species, and have reference to the erect posture.
The inferior aspect of the pelvis presents to view the inferior strait, or outlet of the true pelvis ; which, on account of its more limited extent than the superior outlet, reveals no thing of the interior save the overhanging promontory of the sacrum. It is remarkable in presenting three bony prominences, viz., the two tuberosities of the ischia laterally, and the coccyx postericrly, separated by three notches, placed opposite to each prominence respectively, viz., the sacro-sciatic, postero laterally, and the sub-pubic notch anteriorly. The sacro-sciatic notches being closed by the great sacro-sciatic lig,ament, the completely formed opening thus assumes a lozenge shape, of which the lower part of the pubic symphysis and the tip of the coccyx form the extremi ties of the long diameter ; the tuberosities of the ischia those of the short diameter ; the oblique united rami of the ischia and pubes the antero-lateral, and the great sacro-sciatic ligaments the postero-lateral sides. Of these boundaries it is to be especially remarked, that the coccyx and those parts of the liga ments which are attached to it, are not fixed like all the previously described boundaries of the pelvis, but movable, on the sacro coccygeal articulation, and consequently, the diameters of this outlet dependent upon them, viz., the antero-posterior and the oblique or diagonal, are increased or diminished by the movements of this joint backwards or for wards. The only fixed diameter of the in ferior outlet of the pelvis is the transverse one between the ischial tuberosities. Of the prominent osseous points here seen, the lateral ischial tuberosities descend much lower than the symphysis pubis and coccyx, en ac count of the wavy outline and oblique direc tion of the innominate bones. It is upon these tuberosities only, consequently, that the trunk rests in the sitting posture, and not upon a tripod formed by them and the coccyx, as has been erroneously supposed by some older writers. The boundaries of the inferior outlet, from the same cause, do not, like those of the superior, lie all in one plane or level, but are bent, as it were, at the ischial tuber osities, into two planes ; an anterior, termin ated by,and nearlyin a line with, the symphysis pubis, looking downwards and a little for wards ; and a posterior, terminated by and in cluding the coccyx, directed downwards and backwards, parallel with the superior pelvic plane, but var3 ing with the extension of the coccyx downwards. The plane of this outlet, however, is usually considered to be marked by a straight line joining the lower border of the symphysis pubis and the tip of the coccyx ; and its general direction to coincide with a line drawn perpendicular to this plane down wards and backwards.