About an inch and a half below the inferior curved line is a large articular cavity for the reception of the head of the thigh bone, which is named the acetabulum, or cotyloid cavity,— KorvXn, a cup ; and Woe, like (4). This is a perfectly hemispherical excavation of about two inches diameter in the full grown male, and cir cumscribed by a rough irregularly raised brim or circular border, to which is attached the circumferential fibro-cartilage or cotyloid liga ment of the hip-joint. When the bone is pro perly placed, as in the articulated skeleton, the axis of this cavity is directed outwards arid a little downwards for the better adaptation of the femur; and to this end the posterior part of its circumference is imuch thicker and more prominently elevated than the anterior. Su periorly it is still more evidently prolonged outwards at the point where the before men tioned thick vertical ridge of the ilium springs from it. Inferiorly the border is interrupted for the space of an inch by the cotyloid notch (5), to the edges of which are attached the transverse and interarticular ligaments of the hip-joint, and which is continued into the cen tre of the cavity by a rough depression or fossa, for the reception of some lubricating glands and the interarticular ligament connected with the femur. The bottom of the notch is on the same plane as that of the depression, and affords an entrance for the vessels and nerves supplying the joints. The remainder of the cavity is a smooth and even surface, uniformly concave and circular, which is covered in the fresh state by a cartilage of a semilunar shape. The broadest part of this surface is above where the superior border projects. The pos terior extremity is prolonged into a lip which a little overhangs the notch posteriorly, and terminates exactly opposite to the broadest part of the overhanging superior margin. The anterior extremity is the narrowest, being notched slightly by the groove below the an terior inferior iliac spine on the anterior bor der of the bone before mentioned. At the bottom of the cotyloid fossa may be traced two lines arranged in the shape of a T, the lower limb of which divides the cotyloid notch into nearly equal portions. These mark the fcetal division of the bone into three por tions, of which the ilium contributes the tvvo superior fifths, the ischium the infeiior pos terior two fifths, and the pubis the rfmaining anterior inferior fifth, to the formation of the cotyloid cavity. Externally, the brim of the ace tabula»: is convex, rounded, rough, and marked above and behind by the attachment of the capsular ligament of the hip-joint ( 6). It is here perforated by numerous foramina for the admission of nutritive vessels. It is much better pronounced above and behind, where it presents a broad, thick, convex sur face, than in front where it is shallow, thin, and slightly depressed. The points where this difference is indicated are, the inferior spine of the ilium above, and the cotyloid notch below.
Springing from the cotyloid cavity are two branches of bone ; one from the inferior part, thick, massive, directed downwards and back wards called the descending ramus or body °like ischium(p); and the other, from the anterior part, slighter in structure, and directed obli quely downwards, forwards, and inwards in the same plane with the ilium, called the horizontal ramus or body of the pubis (q). These are each prolonged at their further extremities into flattened tapering processes, which, alter ing the original direction of their respective portions of bone, one ascends obliquely in wards, and the other descends obliquely, out wards, to unite with each other midway', at a point marked by a slight transverse line. They are named respectively the ascending ramus of the ischium (r), and the descending ramus of the pubis (8).4 Together they form the inferior border of the innominate bone, and complete the formation of a large oval opening, situated immediately below, and a little internal to the coryloid cavity, having its long axis directed obliquely downwards and outwards, and called the obturator or thyroid foramen (o)-5upfec, a shield; and like. The edges of this opening are thin, bevilled off, and rough, for the attachment of a fascial ligament which closes the opening, and are formed entirely by the ischium and pubes. The external edge, iustead of meeting the internal superiorly, is continued inwards in front of it, along the superior ramus to the spine of the pubes before described, forming a prominent rib of bone of a triangular shape (q), its base abut ting on the cotyloid cavity. This rib is con
vex vertically, and concave laterally. Between it and the internal edge of the thyroid fora men is left a groove, called the sizb-plibic or obturator groove (t), for the passage of a nerve and vessels, and which has a direction down wards and inwards. The junction of the horizontal mid descending rami of the pubis is called the angle of the pubis, and it is hol lowed, rough, and broad in front, for the attachment of some muscles of the leg. There are ntonerous nutritious openings on this sur face of the ischium and pubis, which are chiefly directed towards the cotyloid cavity.
The internal or pelvic surface presents for examination a superior or iliac portion di rected forwards, upwards, and inwards, and an inferior ischio-pubic portion directed in wards and backwards.
The iliac portion is rough at the posterior third, and is marked by a thick, massy, irre gular prominence, just below the posterior extremity of the crest, which is continued to the posterior superior spine, and serves for the attachment of powerful ligaments which connect this bone to the sacrum. This may be called, for brevity, the iliac tuberosity (1).
An inch and a half below this is an angu lar or semilunar articular surface, the sacral, or auricular (2), to which the sacrum is at tached in the complete pelvis. This surface is generally more or less rough and irre gular, for the more firm attachment of the articular cartilage. It is composed of two elong,ated portions placed at right angles to each other, of which the inferior is the longer, and is directed horizontally backwards to the posterior inferior spine, parallel with and close above the upper boundary of the great sciatic notch (m, n), while the superior is directed vertically upwards towards the crest of the ilium, to which its raised anterior border is prolonged by a well marked ridge (3). At the angle of junction of these two limbs is gene rally to be seen a deep hollow, while the ex tremity of the inferior limb is bevelled off to correspond to elevations on the opposed sur faces of the sacrum. IR the retiring angle formed by the auricular surface is a very rou,gh depression for the attachment of inter-osseous ligaments (7).
The anterior two thirds of the internal iliac surface forms a complete smooth and regular hollow called the internal iliac fossa (4) for the reception of a muscle, u hich is continued downwards and forwards into the groove before described, below the anterior inferior spine of the ilium (d to e). At a small distance in front of the articular surface, is a large nutritious opening, directed down wards and forwards towards the cotyloid cavity, for the passage of the principal nu tritive artery of this bone. Many others of less calibre, all directed downwards, also exist on this surface. Pas,,ing downwards and for wards from the angle of the sacro-iliae arti cular surface is a thick, rounded and arched ridge of bone (5), which serves to transmit the weight of the spine from the sacral arti cular surface to the femur at the cotyloid articulation. Just behind the ilio-pectineal eminence, before described, this tine becomes more salient, and passes from this point to the spine of the pubes as a well marked pro jecting crest, the ilio-pectineal line (6), which serves for the attachment of a muscle of the leg, and some strong faseim. The internal surface of the pubis and ischium, and the small inferior, or true pelvic portion of the ilium, are seen below this line, which separates them from the iliac fossa. Posteriorly is seen a broad, flat surface (p), to which is op posed the cotyloid cavity externally, and the rough lines, marking the junction of the three coniponent bones, are repeated more strongly and more extensively here than in the coty loid cavity itself. They form a Y shape, the posterior limb of which is directed backward to the top of the great sciatic notch ; one branch terminates at the pectineal eminence ; and the other at the obturator opening, where it passes into the commencement of the sub pubic groove (t), the termination of which was described with the external surface. An teriorly is seen the inner aspect of the obtu rator foramen (o), and the posterior surfaces, and slightly marked line of union of the branches of the pubis and ischium (r, s), which are smooth, rounded off, and laterally convex for the apposition of the pelvic viscera. Below is the inner aspect of the great tuber osity of the ischium (i), which is also smooth and concave for the like purposes.