THE SACKUM. 



97 



iliac joint ; this feature is common in the Simiidae and some of the lower races 

 of mankind (Paterson). ,The surface of bone between and lateral to the first, 

 second, third, and fourth foramina affords attachment to the fibres of origin of 

 the piriformis, which may in some instances extend on to the bodies of the second 

 and third segments (Adolphi), whilst on the edge lateral to and below the fourth 

 foramen the coccygeus is inserted. 



The posterior surface is rough and irregular. Convex from above downwards, it 

 displays in the median plane the crista sacralis media, a crest whereon are seen four 

 elongated tubercles the spines of the upper four sacral vertebrae. Lateral to these 

 the bone forms a groove the sacral groove the floor of which is made up of the 

 confluent laminae of the corresponding vertebrae. In line with the intervals between 

 the spines, and wider apart above than below, another series of tubercles is to be 



Superior articular processes Transverse process of first sacral vertebra 



Ala 



Anterior sacral 

 foramen 



Inferior lateral angl 



Groove for fifth sacral nerve 



Coccygeal articular surface 

 FIG. 112. THE SACRUM (anterior view). 



seen. These are due to the fusion of the articular processes of the sacral vertebrae, 

 which thus form faint interrupted ridges on each side of the bone (cristse sacrales 

 articulares). Normally, the spine of the lowest sacral segment is absent, and the 

 laminae do not coalesce medially, thus leaving a gap in which the sacral canal 

 is exposed (hiatus sacralis) ; whilst inferiorly the tubercles corresponding to the 

 inferior articular processes of the last sacral vertebra form little down-projecting 

 processes the sacral cornua by means of which the sacrum is in part united 

 to the coccyx. Just wide of the articular tubercles are the posterior sacral 

 foramina, for the transmission of the posterior rami of the sacral nerves. 

 These are in correspondence with the anterior foramina, so that a probe can be 

 passed directly through both openings; but be it noted that the posterior are 

 much smaller, and their margins much sharper, than is the case with the anterior. 

 The surface of the pars lateralis (lateral mass) lateral to the posterior sacral 

 foramina is rough and irregular, owing to the presence of four more or less elevated 

 tubercles, which constitute the lateral ridges on either side of the bone (cristae sacrales 



7 



