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THE PELVIS. 215 
The inlet (apertura pelvis superior) of the pelvis is bounded in front by the 
symphysis pubis, with the body of the pubis on either side; laterally by the ilio- 
pectineal lines; and behind by the sacral prominence. The circumference of this 
aperture issoften called the brim of the pelvis; in the male its shape is cordate, in 
the female more oval. The antero-posterior or conjugate diameter is measured from 
the sacro-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 point of greatest width. 
The outlet (apertura pelvis inferior) is bounded anteriorly by the pubic arch 
(arcus pubis), formed in front and above by the bodies of the 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 
outwards to the ischial tuberosities, which are placed on either side of this aperture. 
In the middle line and 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 great sacro-sciatic 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 14 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 centre of these planes. Thus, with the 
pelvis in its true position, when the figure is erect, the axis of the inlet corresponds 
to a line drawn downwards and backwards from the umbilicus towards the tip of 
the coceyx below, whilst the axis of the outlet is directed downwards and slightly 
backwards, or downwards and a little forwards, varying according to the length of 
the coceyx. Between these two planes the axis of the cavity, as it passes through 
planes of varying degrees of obliquity, describes a curve repeating pretty 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 inlet 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 wie 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) 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 con- 
trasted with the male pelvis, which is a long segment of a short cone. The cavity of the 
true pelvis 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 under 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 sacro-sciatic notch in the female is wide 
and shallow. The distance from the posterior edge of the body of the ischium to the 
