212 OSTEOLOGY. 
Flattened and compressed, it unites with the inferior ramus of the ischium, and 
thus encloses the thyroid foramen, whilst in correspondence with its fellow of 
the opposite side it completes the formation of the pubic arch.  <Anteriorly it 
furnishes origins for the gracilis, adductor brevis, and adductor magnus muscles, 
as well as some of the fibres of the obturator externus muscle. Its inner surface 
is smooth, whilst its lower border, rounded or more or less everted, has attached to 
it the fore-part of the crus penis and the sub-pubic ligament. 
The acetabulum or cotyloid cavity is the nearly circular hollow in which the 
head of the thigh bone fits. As has been already stated, it is formed by the fusion 
of the ilium and ischium and pubis in the following proportions: the ilium a little 
less than two-fifths, the ischium somewhat more than two-fifths, the pubis con- 
stituting the remaining one-fifth. It is so placed as to be directed downwards, 
outwards, and forwards, and is surrounded by a prominent margin, to which the 
capsule and cotyloid ligament of the hip-joint are attached. Opposite the 
obturator foramen this margin is interrupted by the cotyloid notch (incisura 
acetabuli); immediately external to the ilio-pectineal eminence the margin is 
slightly hollowed, whilst occasionally there is a feeble notching of the border 
above and behind. These irregularities in the outline of the margin correspond to 
the lines of fusion of the ilium and pubis and the ilium and ischium respectively. 
The floor of the acetabulum is furnished with a horseshoe-shaped articular surface, 
which lines the circumference of the hollow, except in front, where it is interrupted 
by the cotyloid notch. It is broad above; narrower in front and below. Within 
this articular surface there is a more or less circular rough area (fossa acetabuli) 
continuous in front and below with the floor of the cotyloid notch. This, some- 
what depressed below the surface of the articular area, lodges a quantity of fat, 
and provides accommodation for the interarticular ligament of the joint. As may 
be seen by holding the bone up to the light, the floor of this part of the acetabulum 
is not usually of great thickness. The major part of the non-articular area is 
formed by the ischium, which also forms the floor of the cotyloid notch. 
The thyroid or obturator foramen (foramen obturatum) hes in front of, 
below, and internal to the acetabulum. The margins of this opening, which are 
formed in front and above by the pubis, and behind and below by the ischium, are 
sharp and thin, except above, where the under surface of the superior ramus of the 
pubis is channelled by the obturator groove. Below, and on either side of this 
groove, two tubercles can usually be seen. The one, situated on the edge of the 
ischium, just in front of the cotyloid notch, is named the posterior obturator 
tubercle (tuberculum obturatorium posterius); the other, placed on the lower 
border of the inner surface of the superior ramus of the pubis, is called the 
anterior obturator tubercle (tuberculum obturatorium anterius). Between these 
two tubercles there passes a ligamentous band, which converts the groove into a 
canal along which the obturator vessels and nerve pass. Elsewhere in the fresh 
condition the obturator or thyroid membrane stretches across the opening from 
margin to margin. The form of the foramen varies much, being oval in some 
specimens, in others more noerly triangular; its relative width in the female is 
greater than the male. 
Nutrient foramina for the ilium are seen on the floor of the iliac fossa, just in front of the 
sacro-auricular surface ; on the pelvic aspect of the bone, close to the great sciatic notch ; and on 
the gluteal surface externally, near the centre of the middle curved line. For the ischium, on its 
pelvic surface, and also externally on the groove below the acetabulum. For the pubis, on the 
surface of the body, and deeply also from the acetabular fossa. 
Connexions.—The innominate bone articulates with the sacrum behind, with the femur to 
the outer side and below, and with its fellow of the opposite side internally and in front. Each 
of its three parts comes into direct relation with the surface. Above, the iliac crest assists in 
forming the iliac furrow, which serves to separate the region of the flank from that of the 
buttock. In front, the anterior superior iliac spine forms a definite landmark ; whilst behind, 
the posterior superior iliae spines will be found to correspond with dimples situated on 
either side of the middle line of the root of the back. The symphysis, the crest, and spine of the 
pubis can all be distinguished in front, though overlain by a considerable quantity of fat, 
whilst the position of the tuberosities of the ischia, when uncovered by the great gluteal 
muscles in the flexed position of the thigh, can readily be ascertained. In the perineal region 
the outline of the pubic and ischial rami can easily be determined by digital examination. 
