THE SACRUM. 



125 



essential modification, besides the fusion, is the occurrence of the lateral masses,^ 

 representing transverse processes and ribs, which, springing from the bodies and 

 arches, are connected with the innominate bones by joints and hgaments. The 

 sacrum has an upper surface, or base, a lower, or apex, and a front, back, and two 

 lateral surfaces. The base has above a rough space representing the end of the 

 body of a vertebra to which the last lumbar disk is attached. It is raised a little 

 from the bone and forms an acute projecting angle with the front surface, known as 

 the promontory of the sacrum, an important landmark in midwifery. Behind the 

 body of the first sacral vertebra is the triangular orifice of the sacral canal, the 



Articular process 



Fig. 153. 



Lamina Sacral canal 



Transverse process of 

 first sacral vertebra 



Posterior- 

 sacral 

 foramen 



Rudimen 

 tary articu- 

 lar pro- 

 cesses 



Spinous 

 process 



Rudimen- 

 tary trans- 

 verse pro- 

 cesses 



Gluteus maximus 



Sacral cornu 



Sacral canal 

 The sacrum, posterior surface. 



transverse diameter of which is the greater. The articular process, springing from 

 the side of the arch, is vertical, the concave facet facing backward and inward. The 

 upper surface of the lateral mass, the ala, springs from the side of the body and 

 the pedicle, expanding into a broad area, and is bounded in front by an ill-marked, 

 rounded border which separates it from the anterior surface and curves forward ; 

 behind by a shorter border curving backward, on which the auricular process rests ; 

 and outside by an irregular convex border. The latter may often be subdivided 

 into two parts : an anterior, running pretty nearly forward and backward and cor- 

 responding to the top of the auricular surface, and a posterior, running backward 



^ Partes laterales. 



