Anterior Face.—This is slightly concave, and recalls that of the sacrum, by the presence of projecting lines separating quadrangular spaces, which diminish from the base to the apex of the bone.
Posterior Face.—This is convex, rough, and is only separated from the skin of the coccygeal region by ligamentous fibres.
Edges.—The edges are rough, and give attachment to the ischio coccygeal muscles, and to the sacro-sciatic ligaments.
Base.—The base points upward, and presents an elliptical concave facet which articulates with the apex of the sacrum. Behind this facet are two projections which form the cornua of the coccyx.
Apex.—The sacrum and coccyx, placed below the vertebral column, the terminal prolongation of which they appear to be, are, in infancy, formed of separate pieces, which resemble the vertebrae. The pieces unite at about the 8th—lOth year, and become firmly welded between the 15th —18th.
The or Os Innominalam.
The iliac bone, os innominatum, coxal-bone, or haunch bone, is a double, non-symmetrical bone which forms the front and side walls of the pelvis.
Formed by the union of three bones, the ilium, the pubis, and the ischium, which do not become welded together till 15 or 16 years of age, it presents an extremely irregular aspect. It has a quadrilateral shape, in which two parts can be distinguished: an upper part, flattened from within outwards; a lower part, flattened from in front backwards.—Two faces and one circumference can be distinguished.
External external or femoral face has, in its centre, the cotyloid cavity, which is deep, hemispherical, and, for the larger part, smooth. - Its edge is thin, sharp and sinuous, with two depressions and a notch. The two depressions correspond to the tendons of the pyramidalis and psoas and iliacus muscles. Through the notch, which by a ligament is formed into a foramen, pass the vessels.
The cotyloid cavity receives the head of the femur. The ligament= teres is attached to its base. It faces downward, outward and a little forward.
Above the cotyloid cavity is a large surface, concave on the back, con vex in front. This is the external iliac fossa, which is crossed by two slightly projecting crests, to which the gluteal muscles are attached.
The external face shows the obturator or sub-pubic foramen, below the cotyloid cavity. This is nearly triangular in shape, presenting, above and outside, a groove through which pass the sub-pubic nerves and vessels. This is the obturator groove. The foramen is closed by a fibrous mem brane—the obturator membrane. It is limited above by a convex pris matic surface, the horizontal ramus of the pubes; on the inner side and above by a quadrilateral surface, the body of the pUbes ; on the inner side and below by another quadrilateral surface, which is long, and runs from above downward, and from the inner side outward. This is the ramus of the ischium. Finally, the foramen is closed below and externally by a larger mass, the tuberosity of the ischium, which ends the body of the ischium. Between the cotyloid cavity and the body of the ischium is a groove for the passage of the tendon of the external obturator.
Internal presents near its centre a curved line, the linea innominata dividing the face into two parts, the superior and the inferior, and forming part of the superi6r strait. The line is limited in front by the ilio-pectinal eminence, behind by the sacroiliac symphysis. That portion of the internal face of the iliac bone which is above the linea innominata has two parts. The first, oblique and concave, facing down wards, is smooth, and is the internal iliac fossa. The other is itself divided into two parts, one the articular part, which articulates with the corresponding surface of the sacrum, the other, uneven and rough, to which strong ligaments are attached. Upon this rough part there is a projecting point which is received into a corresponding depression of the sacrum.
Below the linea innominata, are found: 1st, The posterior'face and the horizontal ramus of the pubes; 2d, the obturator foramen and its groove; 3rd, the posterior face of the ascending ramus of the ischium, of the body. and of the tuberosity of the ischium; 4th, a large quadrilateral surface forming the floor of the cotyloid cavity.