THE PELVIS. 237 



heart-shaped, in the female more oval. The antero -posterior or conjugate diameter 

 is measured from the sa,cro-vertebral angle to the symphysis pubis ; the oblique 

 diameter from the sacro-iliac joint of one side to the ilio-pectineal eminence of 

 the other; whilst the transverse diameter is taken across the greatest width of 

 the pelvic aperture. 



Apertura Pelvis Inferior. The lower opening is bounded anteriorly by the arcus 

 pubis (pubic arch), formed in front and above by the bodies of the ossa pubis, with the 

 symphysis between them, and the inferior pubic rami below and on either side. 

 These latter are continuous with the ischial rami, which pass backwards and 

 laterally to the ischial tuberosities, which are placed on either side of this aperture. 

 In the median plane behind, the tip of the coccyx projects forward ; and in the 

 recent condition the interval between this and the ischial tuberosities is bridged 

 across by the sacro-tuberous ligament, the inferior edge of which necessarily 

 assists in determining the shape of the outlet. 



As the anterior wall of the cavity, formed by the symphysis pubis, measures 

 from \\ to 2 inches, whilst the posterior wall, made up of the sacrum and coccyx, is 

 from 5 to 6 inches in length, it follows that the planes of the inlet and outlet are 

 not parallel, but placed at an angle to each other. The term axis of the pelvis is 

 given to lines drawn at right angles to the centres of these planes. Thus, with the 

 pelvis in its true position, when the figure is erect, the axis of the upper opening 

 corresponds to a line drawn downwards and backwards from the umbilicus towards 

 the tip of the coccyx below, whilst the axis of the lower opening is directed down- 

 wards and slightly backwards, or downwards and a little forwards, varying 

 according to the length of the coccyx. Between these two planes the axis of the 

 cavity, as it passes through planes of varying degrees of obliquity, describes a curve 

 repeating fairly closely the curve of the sacrum and coccyx. 



Position of the Pelvis. The position of the pelvis in the living, when the figure is 

 erect, may be approximately represented by placing it so that the anterior superior iliac 

 spines and the symphysis pubis lie -in the same vertical plane. Under these conditions 

 the plane of the upper opening is oblique, and forms with a horizontal line an angle of from 

 50 to 60. The position of the pelvis depends upon the length of the ilio-femoral ligaments 

 of the hip-joint, being more oblique when these are short, as usually happens in women in 

 whom the anterior superior iliac spines tend to lie in a plane slightly in advance of that 

 occupied by the symphysis pubis. In cases where the ilio-femoral ligament is long a 

 greater amount of extension of the hip-joint is permitted, and this leads to a lessening of 

 the obliquity of the pelvis. This condition, which is more typical of men, results in the 

 anterior superior iliac spines lying in a plane slightly posterior to the plane of the sym- 

 physis, whilst the angle formed by the plane of the inlet and the horizontal is thereby 

 reduced. Bearing in mind the oblique position of the pelvis, it will now be seen that the 

 front of the sacrum is directed downwards more than forwards, and that the sacral pro- 

 montory is raised as much as from 3 to 4 inches above the upper border of the symphysis 

 pubis, lying higher than the level of a line connecting the two anterior superior iliac spines. 

 From the manner in which the sacrum articulates with the ilia, it will be noticed that the 

 weight of the trunk is transmitted downwards through the thickest and strongest part of 

 the bone (see Architecture, Appendix A) to the upper part of the acetabula, where these 

 rest on the heads of the femora. 



Sexual Differences. The female pelvis is lighter in its construction than that of 

 the male ; its surfaces are smoother, and the indications of muscular attachments less 

 marked. Its height is less and the splay of its walls not so pronounced as in the male, 

 so that the female pelvis has been well described as a short segment of a long cone as 

 contrasted with the male pelvis, which is a long segment of a short cone. The cavity of 

 the pelvis minor in the female is more roomy, and the ischial spines not so much inturned. 

 The pubic arch is wide and rounded, and will usually admit a right-angled set-square 

 being placed within, so that the summit touches the inferior surface of the symphysis pubis, 

 whilst the sides lie in contact with the ischial rami. In the male the arch is narrow and 

 angular, forming an angle of from 65 to 70. The greater sciatic notch in the female is 

 wide and shallow. The distance from the posterior edge of the body of the ischium to the 

 posterior inferior iliac spine is longer, measuring on an average 50 mm. (2 inches) in the 

 female, as contrasted with 40 mm. (If inches) in the male. The angle formed by the 

 ischial and iliac borders is more contracted and acute in the male as compared with the 



