BONES. 



comparatively far apart. The spine is comparatively short 

 and narrow. [83] 



Sacrum. This is triangular and has an apex, base, two sur- 

 faces, and two sides. The anterior surface is concave vertically 

 and (slightly) transversely. Its central part corresponds to 

 the bodies of the five original vertebrae and has four transverse 

 ridges at their lines of fusion. On either side these ridges end 

 in four foramina (anterior sacral) ; these lie between five bony 

 bars which unite externally in a solid mass (lateral mass). 

 A groove leads outward from each foramen. [83] 



The posterior surface is convex ; mesially it presents a ridge 

 (crest) with four tubercles (rudimentary spines) , and on each 

 side of this a groove (sacral) . On each side, in line with the in- 

 tervals between the spines, and widest apart above, is a row of 

 tubercles (articular processes) . The most inferior of these tu- 

 bercles project downward (sacral cornua) and the laminae are 

 absent (hiatus sacralis) between them. Just external to the 

 articular processes are four foramina (posterior sacral) ; exter- 

 nal to them, on the lateral mass, are four tubercles (transverse 

 processes). [84] 



Base. The body lies centrally and in front; the upper sur- 

 face of the body articulates with the last lumbar vertebra 

 and its anterior margin (promontory) projects. Behind the 

 body is the triangular spinal canal, and behind this the spine. 

 From the side and back of the body, on each side, springs a 

 fan-shaped mass (ala), the upper edge of the lateral mass. 

 Superiorly the ala is concave transversely and convex antero- 

 posteriorly. From the back of the lateral mass on each side, 

 and separated from it by a notch, springs a process (superior 

 articular) with a vertical facet which is concave transversely 

 and looks backward and slightly inward. [85] 



The borders superiorly are thick and present articular sur- 

 faces (iliac articular surfaces) for the ilium, and behind this, 

 on each side, three depressions. Inferiorly they are thin and 

 each ends in an angle (inferior lateral) . The apex is the small 



[183] 



