THE BONES OF THE TRUNK 



33 



interval thus bridged over at either side represents a fifth inter- 

 vertebral foramen, through which the fifth sacral nerve passes. 

 The solid portion external to the posterior foramina at either side 

 is the lateral mass, and it presents a row of four tubercles, each of 

 which is situated external to a posterior foramen. These represent 

 the transverse processes of the lower four sacral vertebrae. The 

 boundaries of the triangular outlet of the sacral canal are the spine 

 of the fourth sacral vertebra above, and the imperfect laminae 

 of the fifth sacral and the sacral cornua at either side. It 



Promontory- 



.'Auricular Surface 



ist Spine 

 Ligamentous Surface 



_ Sacral Coma 



Tip of Coco-x. ^^ ;^^^ - Coccygeal Comn 



Fig. 24. — The Sacrum (Left Lateral View). 



transmits the fifth pair of sacral nerves and the two coccygeal 

 nerves. 



The lateral surface is broad above and narrow below. The upper 

 part is divided into two portions — articular and non-articular. 

 The articular division, anterior in position, is covered by cartilage, 

 and is shaped like an ear, on which account it is called the auricular 

 surface. It articulates with the iliac portion of the os innominatum, 

 and extends over at least the first two sacral vertebrae. The non- 

 articular division, posterior in position, is rough and irregular for 



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