The pelvis is formed by the union of the two hip-bones, one on either side, with the vertebral column, so as to form the pelvic cavity or basin, which is bounded behind by the lower portion of the vertebral column, sacrum and coccyx, laterally by the hip-bones, and in front by the symphysis pubis, the antero-inferior articulation of the two hip-bones together. It completes the trunk and abdomen in their lower parts, and serves to articulate with the thigh-bones (femora) externally. There is a great contrast between the shoulder girdle and the pelvic girdle. The former is freely movable, and has only a slight attachment by the sterno-clavicular joint to the axial skeleton: the latter is immobile, and is firmly adherent to the vertebral column (sacrum) by means of the sacro-iliac joint and powerful accessory ligaments.

The os sacrum (Figs. 5, 8, 32, 35, 38), so called because it was the part of the trunk offered in sacrifice by the ancients to their gods, is formed of five vertebræ (sacral vertebræ) fused together by osseous union, the separate portions of which are easily seen on careful examination. Taken as a whole it forms a pyramid, the base of which (2, Fig. 35) is turned upwards and forwards, corresponding to the body of the first sacral vertebra. This sacral pyramid, being directed obliquely from above downwards and backwards (Figs. 8, 9), presents a surface called antero-inferior, or rather inferior, on which we recognise five united vertebral bodies (Fig. 5, page 28); a postero-superior surface—better called superior—on which we recognise the rudimentary spinous processes (Fig. 9, page 38) and the laminæ of these same five vertebræ, these portions being all united together; and, finally, the lateral surfaces, expanded above to form the auricular surface for articulation with a similar surface on the hip-bone. This surface extends over the sides of the first three sacral vertebræ; and behind it are rough excavations for the attachment of the powerful posterior sacro-iliac ligaments.

The coccyx (21, Fig. 5), placed below the sacrum, is a rudimentary caudal appendage, but instead of being, as in the greater number of mammals, free and movable, it is found in man curved towards the interior of the pelvis, whose inferior outlet it contributes to close. Its borders give attachment to powerful sacro-sciatic ligaments, which help to fill up the space between the vertebral column and the hip-bone on each side.

The coccyx is formed of a series of four vertebræ, welded one with the other, and so diminished in size that each of them is reduced to a small osseous nodule, representing a rudimentary vertebral body.

The hip-bones (ossa innominata) are two in number, one on each side, articulating behind with the sacrum, and uniting with each other in front (Figs. 35, 38) to form the symphysis pubis. In order to understand the arrangement of the parts of which a hip-bone consists it is necessary to notice that this bone is made up originally, in an infant, of three distinct parts, which are afterwards joined together as age advances; of these parts, the superior is called the ilium; the two others are inferior; that placed in front is called the pubis, and that behind, the ischium (Fig. 33). The junction of the three parts is effected at the central portion of the bone, at the bottom of the great articular cavity of the hip-joint (acetabulum). The names of almost every part of the hip-bone are derived from its three constituent portions—namely, the ilium, pubis, and ischium.

Examining first the external surface of the bone, we notice that its upper part is expanded into a large area called the dorsum ilii (5, Fig. 34), which is crossed by three curved lines limiting the attachments of the gluteal muscles (4 and 5, Fig. 34). Below this area the bone is narrowed and presents a circular cavity (13 and 14, Fig. 34), broad and deep, which has received the name of the acetabulum (a vinegar-cup); its use is to form the articular socket of the hip-joint for the reception of the head of the femur. The margin of this cavity is prominent throughout its entire extent, except below, where it is deficient, and gives rise to the notch of the acetabulum; this notch is a valuable starting-point to settle the natural position either of the iliac bone or of the entire pelvis (Fig. 34). If, in fact, this bone of the pelvis is supposed to belong to a figure in the upright position, the acetabular notch should be directed downwards, as shown in Fig. 34. Below the acetabulum the innominate bone is pierced by a large hole, called the obturator or thyroid foramen (22, Fig. 34), bounded by the following parts: behind, by the ischium (20, Fig. 34); in front and above, by the horizontal ramus of the pubis (18); below, by a bony plate formed by a prolongation of the pubis (19) proceeding to join a corresponding prolongation of the ischium. These united bars of bone are called the descending ramus of the pubis and the ascending ramus of the ischium, and they unite to form with those of the opposite side the pubic arch of the completed pelvis. The internal surface of the innominate bone is divided into upper and lower parts by a diagonal line—the ilio-pectineal line—which separates the iliac portion of the bone from the part formed by the ischium and pubis, and also serves to separate in the complete pelvis the upper abdominal part (false pelvis) from the cavity of the true pelvis below. Above this line is the internal expanded surface of the ilium known as the iliac fossa (10, Fig. 35). At its posterior end is the auricular surface of the ilium, for articulation with the sacrum. Below it are, first, a flat surface corresponding to the floor of the acetabulum, and, lower still, the obturator foramen, bounded as we have already described. The borders of the hip-bone are distinguished (Fig. 34) as superior, anterior, posterior, and inferior. The superior border, called the crest of the ilium (1, Fig. 34), is thick and curved like an italic ∫; it is this which marks on the living model the outline of the hip—namely, the limit between the lateral part of the abdomen and the lateral part of the pelvis. It ends in front in the anterior superior spine (2, Fig. 34), and behind in the posterior superior spine of the ilium. The anterior border begins at the anterior superior iliac spine, and presents in succession, from above downwards, a notch, then a prominence called the anterior inferior spine of the ilium (7, Fig. 34), below which is a groove giving passage to the psoas muscle (see later) bounded on its inner side by the ilio-pectineal eminence; finally, this border is continuous with the horizontal ramus of the pubis, and ends internally in the spine and crest of the pubis (17, Fig. 34). The posterior border of the bone is similarly marked by projections and notches. It is limited above by the posterior superior spine of the ilium (3, Fig. 34), and below by the tuberosity of the ischium; and between these are two smaller projections—the posterior inferior spine of the ilium above, and the spine of the ischium below. The ischial spine serves to divide the border into two unequal notches, of which the superior, the larger, is called the great sciatic notch (12, Fig. 34), and the inferior, the smaller, the lesser sciatic notch. Finally, the inferior border is formed by the rami of the pubis and ischium. We will finish the description of this bone by describing its four angles: the antero-superior (2, Fig. 34) is formed by the anterior superior iliac spine; the antero-inferior (17, Fig. 34) by the body of the pubis, which here presents a rough oval surface of considerable extent which articulates with the pubis of the opposite side to form the symphysis pubis; the postero-inferior angle (20, Fig. 34) is formed by the tuberosity of the ischium; and finally, the supero-posterior, thick and flattened, presents on its outer side the posterior iliac spines, and on its inner side the large rough auricular surface, for articulation with the sacrum.

In order to construct the pelvis, the two hip-bones are joined together (at the symphysis pubis), and also join the sacrum (sacro-iliac synarthrosis) by articulations which have nothing in common with those which we have already studied in the limbs—for example, the shoulder or elbow. In those articulations of the limbs the bones were in contact by smooth surfaces, which glided one upon the other without anything interposed between them; such articulations are characterised by their mobility. On the other hand, the sacrum is united on each side to the hip-bones, and the hip-bones themselves articulate in front with each other by rough surfaces, between which are placed plates, more or less thick, of fibro-cartilage (similar to the intervertebral discs), which, in consequence, do not allow them to glide one on the other, but closely join them together. These articulations, which bear the name of synarthroses, or symphyses (σύν, together; φύομαι, to weld), are remarkable not for mobility, but for rigidity. They are characterised by the possession, between the articulating bones, of plates of fibro-cartilage, which act as tough but elastic buffers. Behind, the two sacro-iliac synarthroses support the sacrum, which is firmly wedged in between the two bones of the hip, and is slung between them, while the strong ligaments placed above the joint permit the sacrum to support the weight transmitted to it by the vertebral column. In front and below, the symphysis pubis (3, Fig. 35), besides having fibro-cartilage placed between the bones and adherent to them, is also strengthened by fibrous bands passing superficially from one bone to the other.

These articulations weld the component parts of the pelvis (the sacrum, with the two hip-bones) into a complete basin; but, at the same time, owing to the elasticity of these joints, the pelvis can resist without injury the shocks which are transmitted to it by the vertebral column and the lower limbs. The sacro-iliac and pubic fibro-cartilages break and check the shock which is produced—when, for example, we jump from a height and alight on the soles of the feet.

Besides the sacro-iliac joint and the articulation of the symphysis pubis, the pelvis also possesses other ligaments accessory or special to the hip-bone. The accessory ligaments are those which strengthen the sacro-iliac joint. Above the articulation are the ilio-lumbar ligament, passing from the transverse process of the last lumbar vertebra to the iliac crest at its posterior end, and the sacro-vertebral ligament, passing from the same transverse process to the lateral mass of the sacrum. Below the joint are the two sacro-sciatic ligaments, which, arising together from the side of the sacrum and coccyx in the form of a broad fibrous band, proceed outwards, and, diverging, are attached, one—the great sacro-sciatic ligament—to the tuberosity of the ischium, the other—the lesser sacro-sciatic ligament—to the spine of the ischium. These ligaments convert the sacro-sciatic notches into foramina, through which important muscles pass; and, but for this fact, these ligaments would not need mention here, as they do not appear superficially, being covered by the mass of the gluteal muscles. The special ligaments of the hip-bone are the triangular ligament, which is a membrane partly filling up the pubic arch; the obturator membrane, which almost completely fills the obturator foramen; and Poupart’s ligament (really derived from the aponeurosis of the external oblique muscle of the abdomen, which stretches between the anterior superior spine of the ilium and the spine of the pubis). It is immediately subcutaneous, and corresponds to the fold of the groin. The subcutaneous fascia is attached to the entire length of the ligament, and so creates a depression extending from the spine of the ilium to the pubis. This fold of the groin marks the limit between the skin of the abdomen and that of the thigh. As the deep fascial envelope of the thigh is attached to Poupart’s ligament in its whole length, it pulls on the ligament so as to make it convex downwards in the extended position of the lower limb.

The whole pelvis is expanded above and narrower below, and the lower part of it is, for the most part, concealed from view in the living model. The lower limbs are attached on each side in such a manner that they approach each other at the lower part of the pelvis, so as to leave between them only a narrow interval—the perineum, which corresponds to the genital organs and the parts between the folds of the buttocks. But the upper outline of the pelvis is clearly marked throughout the whole of its extent: on each side the iliac crests, on the superior borders of the hip-bones, form a slightly undulating line, the middle portion of which is most elevated, while its anterior extremity inclines abruptly downwards to terminate at the anterior superior spine of the crest of the ilium, clearly seen in the model when the skin is not loaded with fat; in front, the pelvis presents an extensive curved outline with its concavity upwards, the central parts corresponding to the symphysis pubis, and the lateral parts formed by Poupart’s ligament on each side. This anterior outline of the pelvis forms the lower limit of the abdominal wall. This helps to give the anterior abdominal wall the form of a shield rounded at both upper and lower ends, a form which the ancients seem to have exaggerated in adopting for the epigastric pit a configuration rounded instead of oval, which is the shape it presents in the skeleton. We have previously (page 54) shown how, in numerous cases, the form adopted by the ancient sculptors is sufficiently justifiable.

After having studied the pelvis with regard to its mechanism and its influence on external form, we ought now to examine it with regard to its proportions—namely, its transverse dimensions; but as the prominence of the hips is formed not only by the superior border of the iliac bones, but also by the great trochanters of the femur, this examination cannot be completed without reference to the relation of the bones of the thigh to the pelvis. For the moment, therefore, we must confine ourselves to the study of the proportions of the pelvis in the male and in the female skeleton.

Generally speaking, the pelvis of the female is broader and shorter than that of the male; in the male (Fig. 35) the superior transverse diameter, which is a line that passes through the most prominent portion of the crest of the ilium of one side to the corresponding part of the other, measures from 10 to 12 inches (on the average 11 inches); whilst in the female this line measures from 10¼ to 13¾ inches (on the average 12 inches). On the contrary, the height of the pelvis in man is about 8 inches, whilst it is not more than 7¼ inches in the female. Again, comparing (Figs. 35 and 38) a male with a female pelvis, we see that the first is narrow, the second comparatively broad between the ischial tuberosities.

If we suppose a line drawn at a tangent to the sides of the pelvis, we see that these lines must be prolonged downwards for a considerable distance in order to meet in the female pelvis, but that in the male pelvis they join at a shorter distance from the pelvic outlet. In order to reduce these facts to a simple formula we may say, therefore, that the pelvis of a male represents a long segment of a short cone, while that of the female represents a short segment of a long cone. The student is referred to Figs. 36 and 37, which show this arrangement by means of a simple diagram.

The particular details of shape which distinguish the pelvis in the two sexes are the following:—The thickness of the walls: in the male the bones of the pelvis are stronger, the iliac crests are thicker, more curved, and less expanded, and the different prominences for the insertions of muscles are better marked. The pubic arch, limited above by the symphysis, and on each side by the descending rami of the pubis, is very broad, and has everted edges in the female, in whom it assumes the form of an elliptical arch, while in the male (compare Fig. 35 and Fig. 38 at 3, 6, 6) this same arch is narrow and raised, and takes the form of a pointed arch. For the same reason the tuberosities of the ischium are wider apart in the female than in the male. The spine of the ischium in the female is turned backwards, and appears less obviously in the pelvic cavity. The obturator foramina are wide and triangular in the female, while they are narrow and oval in the pelvis of the male. The sacrum is relatively shorter and broader in the female than in the male.