It will be more convenient first to describe by itself each of these constituent bones; afterwards to consider them as they unite to form one single bone, the os innominatum.
The ILIUM the largest of the three, presents an external and an internal surface, an anterior and a posterior border and a crest.
The external surface of the ilium is bounded above by the crest; below by the acetabulum; and in front and behind by the anterior and posterior borders. It is alternately convex and concave, broad and smooth, forms the external iliac fossa, and gives attachment to the three glutaei muscles, the minimus, median and maximus.- which make up the full and rounded contour of the hip.
The internal surface is bounded above by the ilio-pectineal line, and before and behind by the anterior and posterior borders. It is divided into two parts. The first or anterior part is a smooth, concave surface of considerable extent, which receives the iliacus internus, and is called the internal iliac fossa or center of the ilium. The second or posterior part is immediately behind the iliac fossa, and is divided into a superior and an inferior portion. The superior part, rough, uneven and irregularly shaped, gives attachment to the sacro iliac ligaments. The inferior or auricular portion is uneven and covered with cartilage, for articulation with the corresponding ear-shaped portion of the lateral border of the sacrum.
The crest or spine of the ilium, longer in the female than in the male, is thick, convex and arched. Its terminations in front and behind, form prominent eminences, which are respectively the anterior superior, and posterior superior spinous processes. The former, or anterior superior spinous process of the ilium, gives attachment to Poupart’s ligament; to the tensor vagina femoris, and to the sartorius muscles. Into the latter, or posterior superior spinous process of the ilium, are inserted strong ligaments, which firmly unite this bone to the sacrum. Beneath these processes are the anterior and posterior inferior spinous processes of either side.
The anterior border of the ilium extends from the anterior superior spinous process of the ilium to the ilio-pectineal eminence. It presents two depressions, which are separated by the anterior inferior spinous process. Through the upper depression, between the superior and inferior spinous processes, is transmitted the external cutaneous nerve, and from it arise some fibres of the sartorius muscle. Through the lower depression, between the inferior spinous process and the ilio- pectineal eminence, pass the iliacus and psoas muscles.
The posterior border of the ilium, shorter than the anterior, presents two deep depressions, separated by a prominent point of bone, the posterior inferior spinous process. The inferior and much larger of these depressions is the great sacro-sciatic notch. The upper portion of this border corresponds to that portion of the posterior surface of the ilium which gives attachment to the sacro-iliac ligaments; the lower portion corresponds to the auricular surface which articulates with the sacrum.
The ilium articulates with the sacrum and unites in bony union with the ischium and pubes.
The ISCHIUM, the second in size and lowest in position of the bony components if the os innominatum, is divided into a base or body and a ramus or branch. The base of the ischium is the thickest part, and forms a larger portion of the acetabulum than does the ilium. From the posterior portion of the bone arises the spinous process of the ischium, which runs backwards and inclines towards the cavity of the pelvis. To this spinous process are attached the internal sacro-sciatic ligaments and from it arises the coccygeus, to be inserted into coccyx. The outlet of the pelvic cavity is sometimes materially affected by the abnormal length inclination of this process; which hence becomes an important element in determining the obstetric capacity of the pelvis itself. The tuberosity of the ischium forms the part on which the body rests in the sitting position. The ramus of the ischium extends from this tuberosity to the ramus of the pubes; its lower border forms part of the inferior outlet of the pelvis, and its inner surface forms a corresponding part of the lower wall of the pelvis.
The ischium is united with the ilium and pubes, in the acetabulum, and by ligaments is firmly bound to the sacrum.
The PUBIS, the smallest of the three divisions of the os innominatum, with its fellow of the other side, forms the public arch or anterior portion of the pelvis. it may be divided into a base or body, a horizontal and a descending ramus. The superior, or horizontal ramus extends from the body of the pubes to the symphysis pubis; the inferior, or descending ramus inclines outwards and downwards from the body, to unite with the ascending ramus of the ischium; while the body or base of the pubes, the central and more expanded portion of the bone, forms a small part of the acetabulum. In the symphysis pubis the pubic bones are not in actual contact, but they are separated by a cartilage of considerable thickness, developed upon the face of each bone, similar to that which occurs in the sacro-iliac symphysis. These cartilages are remarkably adapted to promote the perfect elasticity of the pelvic cavity, and so to provide the utmost possible security for its embryonic contents. In some instances the state of this inter-public cartilage is such as to cause a false joint, and to admit so much lateral movement between the public bones as greatly to impede locomotion.
The acetabulum or cotyloid cavity of the os innominatum, is the circular, cup-shaped depression for the reception of the head of the femur. The ischium forms something more, and the ilium something less than two-fifths each of this depression, and the pubes the remainder. Thus the acetabulum forms the centre of union of the three bones which compose the os innominatum; and these three original bones, which, for distinction’s sake, have thus been separately described, unite to form the os innominatum of either side, which latter must therefore be regarded as one- bone.
The obturator foramen, or foramen ovale, as it is sometimes termed, triangular in the male, but oval in the female, is the large aperture situated between the ischium and the pubes. This foramen, formed on the same principle of the double arch which is seen on a larger scale in the construction of the whole pelvis, combines lightness with strength. The obturator ligament, or membrane, almost entirely covers this foramen, and thus forms a part of the wall of the pelvis. By the bulging out of this elastic membrane, more room is provided for the descending head; and this combination of osseous and membranous parts in the pelvic parietes serves, as the same time, to relieve the child’s head and the soft parts of the mother, within the pelvic cavity, from undue compression.
The os innominatum is formed by the union of the three bones thus separately described; and this bone, considered now as a unit, presents for study an external and an internal face, an anterior, a posterior, a superior and an inferior border.
The external face of the os innominatum is occupied, in its upper and posterior portion, by the external iliac fossa. In its respective convex and concave surfaces, this fossa gives origin, insertion and attachment to the glutaeus maximus, medius and minimus muscles. Anteriorly and superiorly appears the acetabulum or cotyloid cavity. Still more directly in front and beneath is found the obturator foramen, already described as being nearly subtended by the obturator ligament.
The internal face of the os innominatum is occupied, in its superior portion, by the internal iliac fossa. Beneath is found the triangular surface which corresponds to the acetabulum and body of the ischium. Anteriorly appear the inner surface of the obturator foramen and membrane, and the internal faces of the ischio-pubic rami and symphysis pubis.
The anterior border is concave, oblique above and horizontal in front; marked by the anterior superior and anterior inferior spinous processes and ilio-pectineal eminence, and terminated by the spine and angle of the pubis.
The posterior border is irregular in shape, oblique from above downwards and from without inwards; marked by the posterior superior and posterior inferior spinous processes, great sciatic notch, spine of the ischium and lesser sciatic notch, and terminated by the tuberosity of the ischium.
The superior border, or crest of the ilium, is convex and sinuously curved, being bent inwards anteriorly and outwards posteriorly. it is terminated by the anterior superior spinous process in front, and by the posterior and superior spinous process behind.
The inferior border, shorter than either of the others, is marked above by the oval articulating surface which forms the symphysis pubis, below by the tuberosity of the ischium, and between the two we find the ischio-pubic rami.
