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The os innominata (nameless bone, Fig. 2) is of a very irregular figure, and the pair occupy the lateral and anterior parts of the pelvis. The external or femoral surface is turned backwards and downwards, as well as outward; at its superior part, inferiorly it looks downwards. Towards the front, the external face presents the cotyloid cavity, or the acetabulum; a little more in advance and below is the subpubic or obturator foramen, which is nearly closed by the obturator ligament.

Fig. 2. Right os innominatum, external surface.

Fig. 2—Represents the external surface of the right os innominatum. A. The external iliac fossa; B, crest of the ilium; C, anterior superior spine of the ilium; D, anterior inferior spine of the ilium; E, horizontal branch of the pubis; F, posterior superior spine of the ilium; G, posterior inferior spine of the ilium; H, acetabulum; I, ischium; K, obturator foramen. At birth the haunch bone, or os innominata, is composed of three bones connected by cartilage. Fig. 3.

The superior portion of the bone is characterized on its abdominal or internal face by a large excavation called the internal iliac fossa (Fig. 4.) This portion is terminated below by a large rounded and concave line. The inferior (lower) portion presents behind a nearly triangular plane surface; near the middle of this is the obturator foramen, and in front is the internal face of the os pubis.

Fig. 3. Left os innominatum, external surface, etc.

Fig. 3—Left os innominatum, partly ossified. The haunch bone as it exists in the child. A, pubis; B, ilium; C, ischium.

Fig. 4. Right os innominatum, internal surface.

Fig. 4—Right os innominatum, internal surface. A, internal iliac fossa; B, anterior superior spinous process of the ilium; C, crest of the ilium; D, posterior superior spinous process of the ilium; E, posterior inferior spinous process of the ilium; F, articular surface; G, spine of the ischium; H, tuberosity of the ischium; I, obturator foramen; K, ischia pubic ramus; L, crest of the pubis; M, the pectineal eminence.