The posterior surface (fig. a) is rough for muscular attachments, and directed upwards and backwards. It is narrower than the op posing parts of the anterior at the upper part of the bone generally, by rather more than half an inch. According to Mr. Ward, a transverse section of the sacrum, an inch below the base (at the second sacral ver tebra), shows that in this place the posterior surface is wider than the anterior by three sixteenths of an inch, so that the sacral wedge is here reversed in obliquity, which he con siders of importance in resisting anterior dislocation of the sacrum. Above this point, the anterior surface is three sixteenths of an inch wider, and below, it resumes its supe riority in vvidth by four sixteenths. In some cases the back and front are of equal width ; in others the anterior diameter exceeds the posterior throughout.
Its general curvatures are convex, following the concavities of the anterior surface. In the median line are four spinous processes (2), the first of which has been described with the base, connected' by a sharp vertical ridge of bone, and corresponding to the four upper pieces or vertebrre of w hich the sacrum is posed, the whole being called the sacral crest.
The last of these, and sometimes the two lower are divided by a notch (10), which opens u the sacral canal at its inferior termination, where it is much compressed antero-poste riorly. On each side of the sacral crest is a narrow vertical groove, corresponding to the vertebral laminm, and bounded externally by four rough tubercles, the articular (4), the last of which are confounded with the bifurcated inferior spine, and project downwards in two inferior sacral horns (5), which are smoothed into facets posteriorly, to articulate with the coccyx. They correspond to the articular processes of the vertebrm. Immediately ex ternal to them, and on the same level, are the four posterior sacral foramina (6), of irregular size, but much smaller than the anterior, to which they are opposed in situation. The broad surfaces of bone between them present another continuous shallow ertical groove, external to which are three or four tubercles, the transverse (7), arranged vertically parallel with the holes, and corresponding to the tips of the transverse processes of the vertebrae. The highest of these are sometimes smoothed into a facet externally (8), by impinging upon the iliac tuberosity, and the fincrth (9) is al ways the largest and most prominent for the attachment of the superficial posterior sacro iliac ligaments. Close to the lateral boun
dary, opposite the two upper transverse tubercles, are two very rough, digital impres sions for the insertion of powerful posterior sacro-iliac ligaments.
The lateral palaces of the sacrum (fig. B) are broad above, and taper gradually downwards. When opposite the two last sacral vertebrm, they become narrow borders (d). Above, at the three upper vertebrm, they oppose the inner surface of the ilia — below, they form the inner margin of the great sciatic notch. At the upper broader portion these surfaces are bevelled off pos teriorly, the posterior surface of the bone being. at this part narrower than the anterior, and its plane being less distinctly different from that of the lateral surfaces. It is over hung by the tuberosities of the ilia. Close to the upper and anterior margins, occupying the two anterior thirds of the lateral aspect of the base, and extending as far downwards as the third sacral vertebra, at which point the anterior surface of the sacrum becomes, as before mentioned, broader, is a large, angular articular surface, the iliac or auricular (e), de pressed along the centre, and exactly cor responding to the shape and irregular surface of the opposing articular surface of the ilium with which this bone is here jointed. The salient angle corresponds to the rounded an terior border'of the lateral masses of the base, and the retiring angle, to the digital depres sion at the edge of the posterior surface. Two prominent portions may be particularly observed on this articular facet, one at the salient angle (e) on the first sacral vertebra, and another at the termination of the in ferior limb (f) on the third sacral vertebra. They correspond to similar depressions in the opposed iliurn.
The sacrum is traversed longitudinally down the middle, but nearer to the posterior than to the anterior surface, by the inferior termination of the spinal canal, which com municates with the anterior and posterior sacral foramina, the terminal nerves of the cauda equina being contained and distributed within it.
Internal structure of the sacrum.— The in terior is composed of a closely reticulated mass of spongy bone, enclosed in thin, laminated surfaces. For its size, it is the lightest bone in the body, from being made up chiefly of cancellous structure. The laminm, spines, and articular processes are, however, chiefly composed of dense bone.