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The ilium has two surfaces, medial and lateral. The lateral surface is divided into two parts, viz., a lower, acetabular, and an upper, gluteal part. The lower forms a little less than the upper two-fifths of the acetabular hollow, and is separated from the larger gluteal surface above by the upper prominent margin of the articular cavity. The gluteal surface, broad and expanded, is concavo-convex from behind forwards. It is traversed by three rough gluteal (O.T. curved) lines, well seen in strongly developed bones, but often faint and indistinct in feebly marked specimens. Of these the linea glutea inferior (inferior gluteal line) curves backwards from a point immediately above the anterior inferior spine towards the greater sciatic notch posteriorly; the bone between this and the acetabular margin is marked by a rough shallow groove, from which the reflected head of the rectus femoris muscle

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arises. The linea glutea anterior (anterior gluteal line) commences at the crest of the ilium, about one inch and a half behind the anterior superior iliac spine, and sweeps backwards and downwards towards the upper and posterior

TENSOR FASCLE LATE part of the greater sciatic notch. The surface between this line and the preceding furnishes an extensive origin for the gluteus minimus muscle. The linea glutæa posterior (posterior gluteal line) leaves the iliac crest about two and a half inches in front of the posterior superior iliac spine, and bends downwards and slightly forwards in a direction anterior to the posterior inferior spine. The area between this and the anterior gluteal line






is for the origin of the gluteus medius muscle, whilst the rough surface immediately above and behind it is for some of the fibres of origin of the gluteus maximus muscle.

The medial 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 cartilagecoated in the recent condition, and articulates with the sacrum.

This area is said to be proportionately smaller in the female, whilst curving round in front of its anterior margin there is often a groove, for the attachment of the fibres of the anterior sacro-iliac ligaments, called the pre-auricular sulcus. According to Derry this groove is better marked in the female, and may be regarded as characteristic of that sex.

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 interosseous and posterior sacro-iliac ligaments. Above this the bone becomes confluent with the inner lip of the iliac crest, and here it affords an origin to the sacrospinalis and multifidus muscles, and some of the fibres of the quadratus lumborum. The anterior part of the medial aspect of the bone is smooth and extensive; 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 medial side of the ilio- pectineal eminence, which is placed just above and in front of the acetabulum and marks the fusion of the

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ilium with the os pubis. Above this the bone forms the shallow iliac fossa, 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 lateral side and the ilio-pectineal eminence medially, 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 posterior part of the fossa. Below and behind the ilio-pectineal line the medial surface of the ilium forms a small portion of the wall of the pelvis minor; the bone here is smooth, and rounded off posteriorly into the greater sciatic notch, where it becomes confluent with the medial aspect of the ischium. This part of the bone is proportionately longer in the female than in the male, and forms with the ischium a more open angle. Just anterior to the greater 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 posterior part of the hip bone. Superiorly its body (corpus) forms somewhat more than the inferior two-fifths of the acetabulum together with the bone supporting it behind and medially. 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 inferior ramus of the pubis, and encloses the aperture called the obturator foramen.

Superiorly, and on the lateral 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 acetabular notch (O.T. cotyloid notch) 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 obturator externus lies. Beneath this the antero-lateral surface of the superior and inferior rami furnishes surfaces for the attachments of the obturator externus, quadratus femoris, and adductor magnus muscles. The postero-lateral surface of the ischium forms the convex surface on the back of the acetabulum. The medial border of this is sharp and well defined, and is confluent above with the border of the ilium, which sweeps round the greater sciatic notch. From this border, on a level with the lower edge of the acetabulum, there springs a pointed process, the spina ischiadica (ischial spine), to which are attached the sacro-spinous ligament and the superior gemellus muscle. Inferior to this, the postero-lateral surface narrows rapidly, its medial border just below the spine being hollowed out to form the incisura ischiadica minor (lesser sciatic notch). The lower part of this surface and the angle formed by the two rami are capped by an irregularly rough piriform mass called the tuber ischiadicum (ischial tuberosity). This is divided by an oblique ridge into two areas, the upper and lateral for the tendon of origin of the semimembranosus muscle, the lower and medial for the conjoined heads of the biceps and semitendinosus muscles. Its prominent medial lip serves for the attachment of the sacro-tuberous ligament, whilst its lateral edge furnishes an origin for the quadratus femoris muscle; in front and below, the adductor magnus muscle is attached to it.

The medial surface of the body and superior ramus of the ischium form in part the wall of the pelvis minor. Smooth and slightly concave from before backwards, and nearly plane from above downwards, it is widest opposite the level of the ischial spine. Below this, its posterior edge is rounded and forms a groove leading to the lesser sciatic notch, along and over which the tendon of the obturator internus passes. To part of this surface the fibres of the obturator internus are attached, whilst the medial aspect of the spine supplies points of origin for the coccygeus and levator ani muscles, as well as furnishing an attachment to the "white line" of the pelvic fascia. The medial surface of the inferior ramus of the ischium is smooth, and so rounded that its inferior edge tends to be everted. To this, as well as to its margin, is attached the crus penis, together with the ischio-cavernosus, obturator internus, transversus perinei, and sphincter muscle of the membranous urethra. In the female, structures in correspondence with these are found.

The anterior part of the hip bone is formed by the os pubis; it is by means of the union of this bone with its fellow of the opposite side that the pelvic girdle is completed in front.

The pubis (os pubis) consists of two rami-a superior (ramus superior ossis

pubis) and an inferior (ramus inferior ossis pubis). formed by the fusion of these two rami is the body.

The broad part of the bone

The body of the os pubis has two surfaces. Of these the posterior or posterosuperior is smooth, and forms the anterior part of the wall of the pelvis minor; hereto are attached the levator ani muscle and pubo-prostatic ligaments, and on it rests the bladder. The anterior or antero-inferior surface is rougher, and furnishes origins for the gracilis, adductor longus, adductor brevis, and some of the fibres of the obturator externus muscles. The medial border is provided with an elongated oval cartilage-covered surface (facies symphyseos) by means of which it is united to its fellow of the opposite side, the joint being called the symphysis pubis. The superior border, thick and rounded, projects somewhat, so as to overhang the anterior surface. It is called the crest. Medially this forms with the medial border or symphysis the angle, whilst laterally it terminates in a pointed process, the pubic tubercle (O.T, pubic spine). From the crest arise the rectus

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abdominis and pyramidalis muscles, and to the tubercle is attached the medial end of the inguinal ligament. Passing upwards and laterally from the lateral side of the body towards the acetabulum, of which it forms about the anterior fifth, is the superior ramus. This has three surfaces: an antero-superior, an antero-inferior, and an internal or posterior. The antero-superior surface is triangular in form. Its apex corresponds to the pubic tubercle; its anterior inferior border to the crista obturatoria (obturator crest), leading from the pubic tubercle to the upper border of the acetabular notch; whilst its sharp posterosuperior border trends upwards and laterally from the tubercle, and is continuous with the iliac portion of the ilio-pectineal line just medial to the ilio-pectineal eminence, forming as it passes along the superior ramus the pubic portion of that same line (pecten ossis pubis). On this line, just medial to the ilio-pectineal eminence, there is often a short sharp crest which marks the insertion of the psoas minor. The base of the triangle corresponds to the ilio-pectineal eminence above and the upper margin of the acetabular notch below. Slightly hollow from side to side, and convex from before backwards, this surface provides an origin for, and is in part overlain by, the pectineus muscle. The posterior

or postero-superior surface of the superior ramus is smooth, concave from side to side, and slightly rounded from above downwards; by its sharp inferior curved border it completes the obturator foramen, as seen from behind. The antero

inferior surface forms the roof of the broad sulcus obturatorius (obturator groove) which passes obliquely downwards and forwards between the lower margin of the antero-superior surface in front and the inferior sharp border of the posterior or internal surface behind. The inferior ramus of the os pubis passes downwards and laterally from the lower part of the body. Flattened and compressed, it unites with the inferior ramus of the ischium, and thus encloses the obturator 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 medial surface is smooth, whilst its lower border, rounded or more or less everted, has attached to it the anterior part of the crus penis and the arcuate (O.T. subpubic) ligament.

The acetabulum 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 constituting the remaining one-fifth. It is so placed as to be directed downwards, laterally, and forwards, and is surrounded by a prominent margin, to which the capsule and labrum glenoidale of the hip-joint are attached. Opposite the obturator foramen this margin is interrupted by the incisura acetabuli (acetabular notch); immediately lateral 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 acetabular notch. It is broad above; narrower in front and below. Enclosed by articular surface there is a more or less circular rough area (fossa acetabuli) continuous in front and below with the floor of the acetabular notch. This, somewhat depressed below the surface of the articular area, lodges a quantity of fat, and provides accommodation for the intra-articular ligament of the joint (ligamentum teres). As may be seen by holding the bone up to the light, the floor of this part of the acetabulum is usually thin. The major part of the non-articular area is formed by the ischium, which also forms the floor of the acetabular notch.

The foramen obturatum (obturator foramen) lies in front of, below, and medial to the acetabulum. The margins of this opening, which are formed. in front and above by the os pubis, and behind and below by the ischium, are sharp and thin, except above, where the antero-inferior 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 acetabular notch, is named the posterior obturator tubercle; the other, placed on the lower border of the posterior surface of the superior ramus of the os pubis, is called the anterior obturator tubercle. 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 membrane stretches across the opening from margin to margin. The form of the foramen varies much, being oval in some specimens, in others more nearly triangular; its relative width in the female is greater than in the male.

Nutrient foramina for the ilium are seen on the floor of the iliac fossa, just in front of the auricular surface; on the pelvic aspect of the bone, close to the greater sciatic notch; and on the gluteal surface laterally, near the centre of the anterior gluteal line. For the ischium, on its pelvic surface, and also laterally on the groove below the acetabulum. For the pubis, on the surface of the body, and deeply also from the acetabular fossa.

Connexions. The hip bone articulates with the sacrum behind, with the femur to the lateral side and below, and with its fellow of the opposite side medially 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 iliac spines will be found to correspond with dimples situated on either side of the median plane of the root of the back. The symphysis, the crest, and tubercle of

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