The sacrum (Fig. 1) terminates the vertebral column, and is perhaps the most important bone in the pelvis, obstetrically considered, as it enters largely into the various deformities of the pelvis. In the adult it is of a triangular shape, the base of the triangle being above and inclining forwards, the apex below and somewhat backwards; its length is from four to four and a half inches; its breadth about four inches, and the greatest thickness, two and a half inches. The internal surface is concave to the amount of half an inch, crossed by four transverse lines, marking the former division by cartilage; here are four pair of holes, through which pass numerous nervous filaments, which afterwards form part of the great sciatic nerve. [Illustration] Fig. 1– A represents the internal or anterior surface of the sacrum. B B represents the articular processes. C C represents the anterior sacral foramen. D represents the articulating surface. It is placed at the posterior part of the pelvis, where it appears like a wedge forced in between the ossa innominata, immediately below the vertebral column and directly above the coccyx. THE OSSA INNOMINATA. 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. [Illustration: 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. [Illustration: 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. ] [Illustration: Fig. 4. Right os innominatum, internal surface.