PELVIS. 



119 



pieces or vertebrae of which the sacrum is com- The last of these, and sometimes the two lower 

 posed, the whole being called the sacral crest, are divided by a notch (10), which opens u 



Fig. 78. 



A, anterior surface and lose of sacrum ; B, lateral and posterior view of the same bone; c, anterior surface 



and base of coccyx. 



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 laminae, 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 vertebras. 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 vertical 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 fourth (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 latei-al surfaces of the sacrum (fig. 

 B) are broad above, and taper gradually 

 downwards. When opposite the two last 

 sacral vertebrae, they become narrow borders 

 (rf). Above, at the three upper vertebrae, 

 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 ilium. 



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 laminae, spines, 

 and articular processes are, however, chiefly 

 composed of dense bone. 



