PELVIS. 



127 



is commonly expressed as the brim, or su- 

 perior outlet of the true pelvis. 



The false pelvis is formed laterally by the con- 

 cave surface of the internal iliac fossae directed 

 upwards, forwards, and inwards ; and poste- 

 riorly by the lateral masses of the 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 

 lumbar 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 (vt 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 overhangs the 

 cavity of the true pelvis, so as to form a pro- 

 jection called the promontory of the sacrum (/), 

 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 Uio-pectinea, 

 or, by some, the linea innominata. 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 symphysis of the pubis, and by 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 Hie sacrum. 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 symphysis 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 openings, 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- 

 tiling 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 posteriorly, 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 ligament, 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, on ac- 



