210 OSTEOLOGY. 
surface immediately above and behind it is for some of the fibres of origin of the 
gluteus maximus muscle. 
The inner surface of the ilium is divided into two areas which present very 
characteristic differences. The posterior or sacral part, which is rough, displays in 
front a somewhat smooth, auricular surface (facies auricularis) which is cartilage- 
coated in the recent condition, and articulates with the sacrum. Above and behind 
this there is an elevated irregular area, the tuberosity (tuberositas iliaca), which is 
here and there deeply pitted for the attachment of the strong posterior sacro-iliae 
ligaments. Above this the bone becomes confluent with the inner lip of the iliae 
crest, and here it affords an origin to the erector spine and multifidus spine 
muscles. The anterior part of the inner aspect of the bone is smooth and exten- 
sive; it is subdivided by an oblique ridge, called the ilio-pectineal line (linea 
arcuata), which passes forwards and downwards from the most prominent point of 
the auricular surface towards the inner side of the ilio-pectineal eminence which 
is placed just above and in front of the acetabulum, and marks the fusion of the 
ium with the pubis. Above this the bone forms the shallow iliac fossa (fossa 
iliaca), from the floor of which the iliacus muscle arises, whilst leading from the 
fossa, below and in front, there is a shallow furrow, passing over the superior 
acetabular margin, between the anterior inferior iliac spine on the outer side and 
the ilio-pectineal eminence internally, for the lodgment of the tendinous and fleshy 
part of the ilio-psoas muscle. If held up to the light the floor of the deepest part 
of the iliac fossa will be seen to be formed of but a thin layer of bone. A nutrient 
foramen of large size is seen piercing the bone towards the hinder part of the 
fossa. Below and behind the ilio-pectineal line the inner surface of the ilium 
forms a small portion of the wall of the true pelvis; the bone here is smooth, and 
rounded off posteriorly into the ilo-sciatic notch, where it becomes confluent with 
the inner aspect of the ischium. Just anterior to the ilio-sciatic notch there are 
usually the openings of one or two large vascular foramina. From this surface 
arise some of the posterior fibres of the obturator internus muscle. 
The ischium constitutes the lower and hinder part of the innominate bone. | 
Superiorly its body (corpus) forms somewhat more than the inferior two-fifths of 
the acetabulum together with the bone supporting it behind and within. Below 
this the superior ramus passes downwards and backwards as a stout three-sided 
piece of bone, from the inferior extremity of which a compressed bar of bone, called 
the inferior ramus, extends forwards at an acute angle. This latter unites in front 
and above with the descending ramus of the pubis, and encloses the aperture called 
the obturator foramen. 
Superiorly, and on the outer aspect of the ischium, the acetabular surface is 
separated from the bone below by a sharp and prominent margin, which is, 
however, deficient in front, where it corresponds to the cotyloid notch (incisura 
acetabuli) leading into the articular hollow; the floor of this notch is entirely 
formed by the ischium. Below the prominent acetabular margin there is a well- 
marked groove in which the tendon of the obturator externus hes. Beneath this 
the antero-external surface of the superior and inferior rami furnish surfaces for the 
attachments of the obturator externus, quadratus femoris, and adductor magnus 
muscles. The postero-external surface of the ischium forms the convex surface on 
the back of the acetabulum. The inner border of this is sharp and well defined, 
and is confluent above with the border of the ilium, which sweeps round the great 
or ilio-sciatic notch. From this border, on a level with the lower edge of the 
acetabulum, there springs a pointed process, the spine (spina ischiadica), to which is 
attached the lesser sacro-sciatic ligament and the superior gemellus muscle. Below 
this the postero-external surface narrows rapidly, its inner border just below the 
spine being hollowed out to form the small sciatic notch (incisura ischiadica minor). 
The lower part of this surface and the angle formed by the two rami are capped by 
an irregularly rough pyriform mass called the tuberosity (tuber ischii). This 1s 
divided by an oblique ridge into two areas, the upper and outer for the tendon of 
origin of the semimembranosus muscle, the lower and inner for the conjoined heads 
of the biceps and semitendinosus muscles. Its prominent inner lip serves for the 
attachment of the great sacro-sciatic ligament, whilst its outer edge furnishes an 
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