94 PRACTICAL ANATOMY. 



anterior sacral or intervertebral foramina for the transmis- 

 sion of the sacral nerves. They are arranged in pairs. 

 The first pair are the largest ; the others decrease gradu- 

 ally in size from the first to the last. The first and 

 second pairs are ovoid, sometimes reniform, with the 

 long axes vertically ; the superior, inner, and inferior 

 borders are abrupt and rounded. Externally the ap- 

 proach to the foramina is by a broad, shallow groove. 

 The third pair are round. The fourth pair are oval, 

 with the long axis obliquely upward and outward. 

 Running horizontally between the sacral foramina are 

 well-marked ridges, which indicate the original separa- 

 tion of the bone into five pieces. At the junction of the 

 anterior surface and base the sacrum presents a promi- 

 nent, sharp lip, called the promontory, which intensifies 

 the angle formed by the articulation of the sacrum and 

 the last lumbar vertebra. 



The posterior surface is irregular and rough. Run- 

 ning along the median line is a strongly-marked, irregu- 

 lar crest, formed by the incomplete fusion of the 

 rudimentary spinous processes. About an inch on each 

 side are the posterior sacral foramina, arranged in pairs 

 and opposite the anterior. They transmit the posterior 

 sacral branches of the spinal nerves. The posterior 

 sacral foramina are smaller in size than the anterior, but 

 have nearly the same form. The superior pair are over- 

 lapped by a sharp plate of bone along their inner borders, 

 and below terminate in a groove. The posterior surface 

 mainly affords attachment to muscles and ligaments. The 

 superior portion, or base, presents a central articulating 

 surface, the opening of the sacral canal, and at the sides 

 of the articulating base expanded surfaces of bone 

 called the alee. The articulating surface is broad, con- 

 vex anteriorly, flattened posteriorly, surrounded by a 



