127. The sacrum, placed like a wedge between the moveable portion of the spinal column and the lower extremities, forms the posterior boundary of the pelvis. The figure of this bone is triangular (fig. XLV. 5); its anterior surface is concave and smooth, for enlarging the cavity of the pelvis and sustaining the organs contained in it (fig. XLV. 5); its posterior surface is convex, irregular, and rough (fig. XXXV.), giving origin to the great muscles that form the contour of the hip, and to the strong muscles of the back and loins that raise the spine and maintain the trunk of the body erect.
128. The base or upper part of the sacrum receives the last vertebra of the loins on a large and broad surface (fig. XLV. 4), forming a moveable joint; and the degree of motion at this point is greater than it is at the higher points of the spinal column. Firmly united at its sides with the haunch bones, it admits there of no degree of motion.
129. The coccyx, so named from its resemblance to the beak of the cuckoo, when elongated by a succession of additional bones, forms the tail in quadrupeds; but in man it is turned inwards to support the parts contained in the pelvis, and to contract the lower opening of the cavity. By means of a layer of cartilage, the medium by which this bone is connected with the sacrum, it forms a moveable articulation, continuing moveable in men until the age of twenty-five, and in women until the age of forty-five; continuing moveable in women thus long, in order that by yielding to the force which tends to push it backwards during the period of labour, it may enlarge the lower aperture of the pelvis, and so facilitate the process of parturition and diminish its suffering.
130. The lateral boundaries of the pelvis are formed by the ilium, the haunch bone (fig. XLV. 11), and by the ischium, the hip bone (fig. XLV. 12). The ilium forms the lower part of the abdomen and the upper part of the pelvis (fig. XLV. 11); its broad expanded wing supports the contents of the abdomen, and gives attachment to the muscles that form the anterior portion of its walls (figs. XLV. 11, and LXIII. 9); its external convex surface sustains the powerful muscles that extend the thigh; and along its internal surface is the prominent line which marks the brim of the pelvis (fig. XLV. 15), and which divides this cavity from that of the abdomen.