General features.-The hip-bone (figs. 442-445) is a large, irregularly shaped bone, constricted at its center and expanded above and below. Its lateral surface is marked near its middle by a deep, cup-shaped hollow, termed the acetabulum, which forms a secure socket for the rounded head of the femur. Below and in front of the acetabulum the bone is pierced by a large oval, or triangular, gap, termed the obturator foramen. Above the acetabulum the bone forms a wide, flattened plate, with a long, curved upper border, termed the crest of the ilium. With this information the student should have no difficulty in assigning a hip-bone to its correct side.
The hip-bone articulates in front with the corresponding bone of the opposite side and the two bones form the pelvic girdle or girdles of the lower limbs. Each consists of three parts, named the ilium, the ischium and the pubis, which are connected by cartilage in the young subject but are united by bone in the adult ; the union of the three parts takes place in the walls of the acetabulum. The lines of fusion are shown as stippled bands in figs. 443 and 445. The ilium includes the upper part of the acetabulum and the expanded, flattened area of bone above it ; the ischium includes the lower part of the acetabulum and the bone below and behind ; the pubis forms the anterior part of the acetabulum and separates the ilium from the ischium in this situation; in addition, it forms the anterior part of the lower portion of the hip-bone and meets the pubis of the opposite side in the median plane.
The ilium.-General features. The ilium, so named because it supports the flank, possesses two ends and three surfaces. The lower end is the smaller and forms rather less than the upper two-fifths of the articular surface of the acetabulum. ; the upper end is greatly expanded and compressed to form the iliac crest. The surfaces are named the gluteal surface, the sacropelvic surface and the iliac fossa. The gluteal surface is directed backwards and laterally and forms an extensive rough area ; the iliac fossa is smooth and gently hollowed out and occupies the anterior and upper part of the medial aspect of the ilium ; the sacropelvic surface is also placed on the medial aspect and lies behind and below the iliac fossa, separated from it by a ridge, termed the medial border.
The iliac crest forms the upper end of the ilium. It is convex upwards in its general outline, but is sinuously curved, being concave inwards in front and con cave outwards behind. Its anterior and posterior extremities project a little beyond the bone below and are termed respectively the anterior and the posterior superior iliac spines. The anterior superior spine lies at the lateral end of the fold of the groin and can be felt without difficulty in the living subject ; the posterior superior spine cannot be felt but it can be identified readily, because its position is indicated by a small dimple, which lies about 4 cm. lateral to the second spinous tubercle of the sacrum in the upper and medial part of the buttock. Morphologically the crest consists of a long ventral and a shorter dorsal segment. The ventral segment forms rather more than the anterior two-thirds of the crest and is associated with alterations in the form of the ilium which were necessitated by the adoption of the erect attitude ; the dorsal segment forms rather less than the posterior third of the crest and can be identified in all animals. The ventral segment of the crest is bounded by an outer and an inner lip, enclosing a rough intermediate area, which is narrowest at its middle and becomes wider both in front and behind. The tubercle of the crest (fig. 442.) forms a prominent projection on the outer lip about 5 cm. or more behind the anterior superior spine. The dorsal segment presents two sloping surfaces separated by a well-marked ridge, which terminates in the posterior superior spine. The highest point of the crest, which lies just behind its midpoint, is placed on the same level as the fourth lumbar vertebra.
The lower end of the ilium will be described with the acetabulum.
The anterior border of the ilium descends to the acetabulum from the anterior superior spine. Its upper part is rounded and concave forwards ; its lower part presents a roughened projection, termed the anterior inferior iliac spine, which lies immediately above the acetabulum.
The posterior border is irregularly curved (fig. 442). It commences above at the posterior superior spine and runs at first downwards and forwards, with a backward concavity, forming a small notch. At the lower end of the notch the bone presents a wide, low projection, termed the posterior inferior spine, where the posterior border makes a sharp bend. It then runs almost horizontally forwards for about 3 cm. and finally turns downwards and backwards to become continuous with the posterior border of the ischium. As a result the posterior border shows a deep notch, bounded above by the ilium and below by the ilium and ischium, which is termed the greater sciatic notch.
The medial border is placed on the medial aspect of the bone and separates the shallow iliac fossa from the sacropelvic surface. Indistinct near the crest, it is roughened in its upper part, then sharp and clear-cut where it bounds the articular surface for the sacrum, and finally smooth and rounded. The last-named portion forms the iliac part of the arcuate line, which marks the inlet of the true pelvis ; at its inferior end it reaches the posterior part of the iliopubic (pectineal) eminence, which is placed at the union of the ilium with the pubis.
The gluteal surface (fig. 442) is directed backwards and laterally behind, and laterally and slightly downwards in front. It is bounded above by the iliac crest, below by the upper border of the acetabulum and in front and behind by the anterior and posterior borders. The surface, as a whole, is smooth and curved, being convex in front and concave behind, but it is marked by three roughened ridges, termed the posterior, middle and inferior gluteal lines. The posterior gluteal line, which is the shortest of the three, begins above on the outer lip of the crest about 5 cm in front of its posterior extremity and ends below a short distance in front of the posterior inferior spine. Its upper part is usually distinct, but its lower part is ill-defined and frequently absent. The middle gluteal line (anterior gluteal line), which is the longest of the three, begins about the middle of the upper margin of the greater sciatic notch and runs upwards and forwards to become confluent with the outer lip of the crest just in front of the tubercle. The inferior gluteal line, which is rarely a prominent feature, begins a little above and behind the anterior inferior spine and curves backwards and downwards to end near the apex of the greater sciatic notch. Between the inferior gluteal line and the margin of the acetabulum there is a rough, shallow groove on the bone. Behind the acetabulum the lower part of the gluteal surface becomes continuous with the posterior surface of the ischium. The site of union of these two elements is marked by a low convex elevation.
The iliac fossa occupies the anterior and upper part of the medial aspect of the ilium. It is limited above by the iliac crest ; in front by the anterior border, and behind by the medial border, by which it is separated from the sacropelvic surface. The surface is smooth and gently concave and forms the lateral wall of the false pelvis. Below it is continuous with a shallow groove (fig. 444) bounded laterally by the anterior inferior spine and medially by a low rounded elevation which marks the fusion of the ilium with the pubis and is termed the iliopubic (pectineal) eminence.
The sacropelvic surface (fig. 444) occupies the posterior and lower part of the medial aspect of the ilium. It is bounded behind and below by the posterior border, in front and above by the medial border, and above and behind by the iliac crest. It is subdivided naturally into three areas, viz. the iliac tuberosity, the auricular surface and the pelvic surface. The iliac tuberosity forms an extensive, roughened, tuberculated and pitted area which lies immediately below the dorsal segment of the iliac crest. It gives attachment to the strong ligaments on the posterior aspect of the sacro-iliac joint. The auricular surface (fig. 444) is placed immediately below and in front of the tuberosity, and articulates with the lateral mass of the sacrum. It is shaped like the auricle, the wide expanded portion lying above and in front and the lobule below and behind, covering the medial aspect of the posterior inferior spine. The edges are sharp and clearly defined, but the surface, although articular, is finely roughened and irregular. The pelvic surface lies below and in front of the auricular surface and helps to form the wall of the true pelvis. It consists of an upper and a lower portion. The upper portion faces downwards and lies between the margin of the auricular surface and the upper border of the greater sciatic notch.
Particular features.–The iliac crest form s the lower limit of the flank and gives attachment to the lateral muscles of the abdominal wall, to fasciae and muscles of the lower limb and to muscles and �asei2e of the back {figs. 443, 445). The outer lip of the ventral segment gives attachment to the fascia lata., including the iliotibial tract ; in front of the tubercle of the crest it gives origin to the tensor fasciae lata ; in its anterior two-thirds it provides insertion for the lower fibers of the external oblique ; and just behind its highest point it gives origin to the lowest fibers of the latissimus dorsi. An interval of variable size intervenes between the posterior limit of the insertion of the external oblique and the anterior limit of the origin of the latissimus dorsi in this situation the crest forms the base of the lumbar triangle. The intermediate area gives origin to the internal oblique muscle. The inner lip in its anterior two-thirds gives origin to the transversus muscle : and behind that to the lumbar fascia and the quadratus lumborum muscle. The dorsal segment (p. 366) gives origin by its lateral sloping aspect to the highest fibers of the gluteus maximus, and by its medial sloping aspect to the sacropsinalis muscle.
The upper part of the posterior border gives attachment to the upper fibers of the sacrotuberous ligament. In front of the posterior inferior spine (i.e. on the upper border of the greater sciatic notch), it gives origin to fibers of piriformis and, in front of that, is related to the superior gluteal vessels and nerve as they emerge from the pelvis. The lower part of the posterior border (i.e. the lower margin of the greater sciatic notch) is covered by the piriformis muscle and is related to the sciatic nerve, although the latter structure lies for the most part on the ischium.
The gluteal surface is divided into four areas by the three gluteal lines (fig. 442). (a) The area behind the posterior gluteal line gives origin in its upper roughened part to the upper fibers of the gluteus maximus ; its lower smooth part gives attachment to some of the fibers of the sacrotuberous ligament. (b) The area between the posterior and middle gluteal lines, bounded above by the iliac crest, gives origin to the gluteus medius muscle. (c) The area between the middle and inferior gluteal lines gives origin to the gluteus minimus muscle. (d) The area below the inferior gluteal line is marked by numerous vascular foramina. The groove above the acetabulum gives origin to the reflected head of the rectus femoris, and the area adjoining the rim of the acetabulum affords attachment to the capsular ligament of the hip-joint. The greater part of this area is covered by the gluteus minimus muscle, but behind and below in the neigbbourhood of the site of union of the ilium and ischium, the bone is related to the piriformis muscle.
The iliac tuberosity on the sacropelvic surface gives attachment to the strong and thick posterior sacro-iliac ligaments and, just behind the auricular surface, to the interosseous sacro-iliac ligament. The upper and anterior part of the tuberosity gives attachment to the iliolumbar ligament, and this area lies immediately below the medial part of the origin of the quadratus lumborum from. the iliac crest. The auricular surface articulates with the upper two and a half sacral vertebrae, (two only, as a rule, in the female). Its sharp anterior and inferior borders give attachment to the anterior sacro-iliac ligament. The upper part of the pelvic surface, between the inferior margin of the auricular surface and the upper margin of the greater sciatic notch, is often marked in female subjects by a roughened groove, which is termed the pre-auricular sulcus ; it gives attachment to the lower fibers of the anterior sacro-iliac ligament. Lateral to the sulcus the bone gives origin to fibers of the piriformis muscle. The rest of the pelvic surface gives origin to the upper half or less of the obturator internus muscle.
The pubis.-General features. The pubis forms the anterior part of the hip-bone and meets the pubis of the opposite side in the median plane to form a cartilaginous joint, termed the pubic symphysis. It possesses a body,* which lies anteriorly ; a superior ramus, which passes upwards and backwards to the acetabulum ; and an inferior ramus, which passes backwards, downwards and laterally to unite with the ramus of the ischium on the medial side of the obturator foramen.
The body is compressed from before backwards and presents anterior, posterior and symphyseal, or medial, surfaces and a free upper border, termed the pubic crest. The anterior surface faces downwards, forwards and somewhat laterally in the erect posture ; rough in its upper and medial parts, it presents a smooth surface elsewhere. It is directed towards the lower limb and affords attachment for the medial group of muscles of the thigh. The posterior surface is smooth and faces upwards and backwards, forming the anterior wall of the true pelvis ; it is related to the urinary bladder. The symphyseal surface forms an elongated oval area, covered with cartilage in the recent state and articulating with the opposite pubis at the pubic symphysis. When denuded of cartilage it presents an irregular surface, marked by a number of small ridges and furrows or by small nodular elevations. The pubic crest forms the rounded upper border of the body. It is projected forwards and overhangs the upper part of the anterior surface (fig. 442). Its lateral extremity is marked by a rounded projection, termed the pubic tubercle. Both the crest and the tubercle can be felt through the skin in the living subject, but the latter is obscured by the spermatic cord, which crosses its upper aspect as it passes upwards from the scrotum to pierce the abdominal wall.
The superior ramus of the pubis springs from the upper. part of the lateral aspect of the body, and passes backwards, upwards and laterally above the obturator foramen to reach the acetabulum. It is triangular on section and possesses three surfaces and three borders. The pectineal surface is directed forwards and slightly upwards. Triangular in outline, it extends from the pubic tubercle to the iliopubic (pectineal) eminence and presents a concave surface (fig. 442). It is bounded in front by a rounded ridge, termed the obturator crest, and behind by the sharp pectineal line, which together with the pubic crest constitutes the pubic part of the arcuate line. The pelvic surface, directed upwards, backwards and medially, forms a smooth surface which is narrower at its lateral than at its medial extremity, where it is continuous with the posterior surface of the body. It is bounded above by the pectineal line and below by a sharp edge which forms the inferior border. The obturator surface is directed downwards and backwards, and is crossed from behind forwards and downwards by a groove, termed the obturator groove. It is bounded in front by the obturator crest and behind by the inferior border.
The inferior ramus springs from the inferior aspect of the body and passes backwards, downwards and slightly laterally to unite with the ramus of the ischium on the medial side of the obturator foramen. The site of union may be marked by a localized thickening, but is often difficult to identify in the adult bone. The ramus possesses two surfaces and two borders. The anterior or outer surface is continuous above with the anterior surface of the body ; it is directed towards the thigh and is roughened for muscular attachments. It is bounded behind by the margin of the obturator foramen and in front by a rough anterior border. The posterior or inner surface is continuous above with the posterior aspect of the body, and is convex from side to side. Its medial part is often prominently everted in male subjects (fig. 452) and is in contact with the crus of the penis. It is directed medially towards the perineum. Its lateral part is smooth and is directed upwards towards the pelvis.
Particular features.-The pubic tubercle gives attachment to the medial end of the inguinal ligament ; it lies in the floor of the superficial inguinal ring (subcutaneous inguinal ring) and is crossed by the spermatic cord. The ascending limbs of the loops of the are master muscle are attached to the tubercle and to the anterior wall of the sheath of the rectus abdominis muscle. The lateral part of the pubic crest gives origin to the lateral head of the rectus abdominis and the bone below to the pyramidalis. The medial part of the crest is crossed by the medial head of the rectus abdominis, which takes origin from an interlacement of fibers in front of the upper part of the pubis and pubic symphysis. The anterior surface of the body is directed towards the adductor region of the thigh. A roughened strip, usually wider in the female marks the medial part of the surface and gives attachment to the anterior pubic ligament. In the angle between the upper end of this strip and the pubic crest the rounded tendon of the adductor longus takes origin. At a slightly lower level the gracilis arises from a linear origin close to the medial border of the body and extending downwards on to the inferior ramus. Lateral to the gracilis the adductor brevis arises from the body and the inferior ramus. The lateral part of the anterior surface, and the adjoining portions of both rami provide origin for the obturator internus muscle (fig. 443).
The posterior surface of the body is separated from the urinary bladder by the retropubic pad of fat. About its middle it provides origin for the anterior fibers of the levator ani muscle, and more laterally the obturator internus arises from this surface and extends on to both rami. Medial to the origin of the levator ani the puboprostatic ligaments are attached to the bone.
The pectineal surface of the superior ramus, along its upper part, gives origin to the pectineus muscle, which covers but is not attached to the rest of the surface (fig. 443). The pectineal line, which forms the upper boundary of the pectineal surface, is a salient, sharp ridge. At its medial end it gives attachment to the conjoint tendon (falx inguinalis) and the pectineal part of the inguinal ligament (lacunas ligament), and throughout the rest of its extent it affords attachment to a. strong fibrous band, termed the pectineal ligament (p. 552). About its middle it receives the insertion of the psoas minor. The pelvic surface is smooth and is uncovered by muscle or fascia. It is intimately related to the parietal peritoneum, and in the intervening subperitoneal tissue the lateral umbilical ligament (obliterated hypogastric artery) runs downwards and forwards across the ramus and, near its lateral end, the vas deferens passes backwards. The obturator groove on the obturator surface is converted into a canal by the upper borders of the obturator membrane, the obturator internus and the obturator externus muscles. It transmits the obturator vessels and nerve from the pelvis to the thigh, where they emerge under cover of the pectineus muscle. The obturator crest (fig. 442) at its lateral end gives attachment to some of the fibers of the pubofernoral ligament.
The outer surface of the inferior, ramus gives origin to the gracilis, the adductor brevis and the obturator externus, from The medial to the lateral side. In addition the origin of the adductor magnus usually extends from the ramus of the isehium on to the lower part of the inferior ramus of the pubis in the interval between the adductor brevis and the obturator externus. The inner surface is divided into a medial, an intermediate and a. lateral area, but they are not separated from one another by clear-cut markings on the bone. The medial area faces downwards and medially and is in direct contact with the crus penis ; it is limited above and behind by an indistinct ridge which gives attachment to the perineal membrane (inferior fascia of the urogenital diaphragm). The intermediate area gives origin to the sphincter urethne muscle. The lateral area gives origin to fibers of the obturator internus muscle. The medial margin of the ramus is strongly everted in the male and gives attachment to the fascia lata, and to the membranous layer of the superficial fascia of the perineum.
The ischium.-General features.-The ischium forms the lower and posterior part of the hip-bone. It consists of a body and a ramus. The body possesses upper and lower extremities, and femoral, dorsal and pelvic surfaces.
The upper extremity forms the lower part of the acetabulum and is fused with the ilium and pubis. The lower extremity is free and is capped by the lower part of a large bony prominence, termed the ischial tuberosity. The ramus springs from the front of the lower part of the body.
The femoral surface of the body is directed downwards, forwards and laterally towards the thigh. It is roughened and uneven, and is bounded in front by the margin of the obturator foramen, and laterally by the lateral border, which is indistinct above but clearly defined below, where it forms the lateral border of the ischial tuberosity. The dorsal surface is directed backwards, laterally and upwards. Above it is continuous with the lower part of the gluteal surface of the ilium, and where the two elements meet the bone presents a low convexity, which corresponds with the curvature of the posterior part of the acetabulum. Below, the surface is covered by the upper part of the ischial tuberosity. Above the tuberosity the bone presents a wide and shallow groove both on the lateral and on the medial side. The ischial tuberosity is a large, roughened impression which covers the lower part of the posterior surface and the inferior extremity of the body of the ischium. It is an elongated area, widest near its upper end and tapering away inferiorly, which provides attachment for some of the large muscles of the thigh. The posterior surface is placed between the lateral and the posterior border of the body, The posterior border is continuous above with the posterior border of the ilium and helps it to complete the lower margin of the greater sciatic notch. The posterior end of that margin is marked by a somewhat pointed projection, termed the ischial spine. Below the spine the border becomes rounded and indefinite, forming the floor of a rounded notch, termed the lesser sciatic notch, which lies between the ischial spine and the tuberosity. The pelvic surface is smooth and relatively featureless and is directed towards the pelvic cavity; its lower part forms the lateral wall of the ischiorectal fossa in the perineum.
Particular features.-The anterior surface of the body of the ischium gives origin below to a part of the obturator externus muscle and along the lateral border of the upper part of the ischial tuberosity to the quadratus fern oris. The upper part of this surface is pitted by numerous vascular foramina and is covered by the tendon of the obturator externus as it passes laterally and backwards in the groove between the acetabulum and the ischial tuberosity. Just below the acetabulum the lateral border gives attachment to the ischiofemoral ligament.
Immediately above the ischial tuberosity the posterior surface is crossed by the tendon of the obturator internus and the gemelli muscles ; the nerve to the quadratus femoris intervenes between these structures and the bone as it descends to reach its destination. At a higher level the bone is covered by the piriformis muscle, which is partially separated from it by the sciatic nerve and the nerve to the quadrates femoris. The ischial tuberosity is divided by a nearly transverse ridge into an upper and a lower area. The upper area is associated with the hamstring muscles ; it is divided by an oblique line into an upper and lateral part which gives origin to the semimembranosus, and a lower and medial part from which the long head of the biceps femoris arises in common with the semitendinosus muscle. The lower portion of the tuberosity narrows as it passes forwards on to the lower end of the ischium. It is divided into a lateral and a medial area ; the lateral area is the larger and affords origin to the adductor magnus muscle; the medial area is covered by fibro-fatty tissue, which usually contains the ischial bursa of the gluteus maximus. It is the medial area on the lower part of the tuberosity which supports the body in the sitting posture. On its medial side the tuberosity is limited by a curved ridge which extends forwards on to the ramps of the ischium and gives attachment to the sacrotuberous ligament and its falciform process (fig. 445). Many of the fibers of origin of the biceps femoris can he traced into the sacrotuberous ligament, and this intimate relationship is noteworthy, fox the sacrum and the posterior part of the ilium constitute the primitive mammalian origin of the biceps femoris. The origin of the muscle from the tuberosity in man is secondary and the saerotuberous ligament represents the remains of its primitive tendon of origin.
Above and medial to the. tuberosity the posterior surface presents a wide, shallow groove.. In this situation the bone is usually covered by a thin layer of cartilage in the recent state and a. bursa is interposed between it and the tendon of the obturator externus, which lies in the groove. The lower margin of the groove, close to the tuberosity, gives origin to the inferior gemellus ; the upper margin of the groove, close to the ischial spine gives origin to the superior gemellus.
The ischial spine projects downwards and medially. Its margins give attachment to the sacrospinous ligament, which separates the greater from the lesser sciatic foramen (Fig. 545). Its dorsal surface is crossed by the internal pudendal vessels and the nerve to obturator internus as they lie in the gluteal region. The ventral surface of the spine gives origin to the coccygeus and the most posterior fibers of the levator ani muscle. The structures transmitted by the greater and lesser sciatic foramina are described on p. 474.
The pelvic surface of the body of the ischium is smooth. Its upper part gives origin to the obturator internus muscle; the fibers of which converge on the lesser sciatic notch and cover the remainder of this surface, with the exception of the pelvic aspect. of the. ischial spine. The muscle and its covering fascia separate the bone from the ischiorectal fossa.
The anerior sacrface of the ramps of the ischium is directed towards the adductor region of the thigh. It gives origin to the obturator externus above. the anterior fibers of the adductor magnus and. near the lower border, to the gracilis muscle. Between the adductor magnus and the gracilis the origin of the adductor brevis may extend downwards from the inferior ramus of the pubis for a short distance. The posterior surface is divided into pelvic and perineal areas. The pelvic area is directed upwards and backwards and gives origin to part of the obturator internus. The perineal area, is directed medially ; its upper part is related to the crus of the penis and gives origin to the sphincter urethra; its lower part gives origin to the ischiocavernosus and to the superficial transverses perinei muscles. The perineal membrane is attached to the ridge which separates the perineal from the pelvic area below and the area for the crux from the origin of the sphincter urethra above. The lower border of the ramus provides attachment for the fascia lata of the thigh and the membranous layer of the superficial fascia of the perineum.
The acetabulum (fig. 442) is a deep cup-shaped cavity on the lateral aspect of the hip-bone about its center, and is directed laterally, downwards and somewhat forwards. It is surrounded by an irregular projecting margin which is deficient inferiorly ; this gap is termed the acetabular notch. The floor of the cavity is roughened and non-articular and is termed the acetabular fossa. The sides of the cup present a horseshoe-shaped articular surface which is widest superiorly ; in this situation the weight of the trunk is transmitted to the femur in the erect attitude. In the recent state this strip is covered with articular cartilage and provides the surface on which the head of the femur moves within the hip-joint. All three elements of the hip-bone contribute to the formation of the acetabulum in man, but not in equal proportions. The pubis forms the upper and anterior fifth of the articular surface ; the ischium, the floor of the acetabular fossa and rather more than the lower and posterior two-fifths of the articular surface ; the ilium forms the remainder of the articular surface.
The obturator foramen is a large gap in, the hip-bone, lying below and in front of the acetabulum and placed between the pubis and the ischium. It is bounded above by the grooved obturator surface of the superior ramus of the pubis ; medially by the body and inferior ramus of the pubis ; below by the ramus of the ischium ; and laterally by the anterior border of the body of the ischium, including the margin of the acetabular notch. The foramen is occupied in the recent state by a fibrous sheet, termed the obturator membrane, which is attached to its margins except above, where a communication is left between the pelvis and the thigh. The free upper edge of the membrane is attached in front to the anterior obturator tubercle, which marks the anterior end of the inferior border of the superior ramus of the pubis, and behind to the posterior obturator tubercle, which is placed on the anterior border of the acetabular notch. These tubercles are not always easy to identify. The foramen is large and oval in the male, but is smaller and somewhat triangular in the female.
Ossification (fig. 447).-The hip-bone is ossified from eight centers : three primary, one each for the ilium, ischium, and pubis ; and five secondary, one each for the crest of the ilium, the anterior inferior iliac spine (said to occur more frequently in the male than in the female), the tuberosity of the ischium, the pubic symphysis (more frequent in the female than in the male), and one or wore for the Y-shaped cartilage at the bottom of the acetabulum. The centers appear its the following order : in the ilium, immediately above the greater sciatic notch, about the eighth or ninth week of fetal life ; in the body of the ischium, about the third month ; in the superior ramus of the os pubis, between the fourth and fifth months. At birth the iliac crest, the greater part of the acetabulum, the ischial tuberosity, and the inferior ramus of the pubis and the ramus of the ischium are cartilaginous. By the seventh or eighth year, the inferior ramous of the pubis and the ramous of the ischium are almost completely united by bone. The three primary centers extend their growth into the bottom of the acetabulum, where they are separated from each other by a Y-shaped portion of cartilage, which begins to ossify by two or more centers in the twelfth year. One of these centers, named the os acetabuli, forms a triangular scale of bone over the acetabular part of the pubis and fuses with the main parts of the bonze about puberty. The ilium and ischium: then become joined, and lastly the pubis and ischium, through the middle of this Y-shaped portion. At about the age of puberty; ossification takes place in each of the remaining portions, and. they join with the rest of the bone between the twentieth and twenty-fifth years. Separate centers are frequently found for the tubercle, crest and angle of the pubis, and the spine of the ischium.
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