THE SACRUM 



15 



THE SACRUM 



This is a large triangular bone, firmly wedged between the innominate bones. 

 It forms the posterior boundary of the true pelvic cavity. The sacrum is curved 

 upitn itself with the concavity looking forwards. The upper end of the curve 

 forms, Avith the body of the fifth luml)ar vertel)ra, an anterior projection known 

 as the promontory. The middle portion of the anterior face of the sacrum 

 exliiltits four transverse ridges corresponding to the intervertebral spaces. The 

 intci'vening juirtions are the bodies of the vertebra\ The upper two sacral verte- 

 brae are almost equal in size to those of the lumbar series, Imt the three lower 

 rapidly diminish in size from above downwards. The ridges terminate laterally 

 in the anterior sacral foramina, four pairs in all, which are the interverte- 

 bral foramina of tlie sacral vertcbne, and transmit the anterior divisions of the first 



Fig. 17.— The Sacrum and Coccvx. (Anterior view.) 



TUBEROSITY 



Pyriformis 



Coccygeus 



Coccygeua 



Levator Ani 



four sacral nerves. The ui)per two are also traversed ])v the lateral saeral 

 arteries. The l)one immediately »^utside the foramina corresponds to the costal 

 processes, and the ])ortion formed by the second, third, and fourth sacral vertel>r;e 

 gives origin to the pijrifonni.-< mi/.-irlr. The lateral i»art of the fifth sacral vertebra 

 gives partial insertion to the rocn/f^en.':. 



The i)Osterior surface is strongly convex and rougli. Tlie midille line is 

 occujned by four tubercles representing the suppressed spinous processes. Of 

 these the first is the largest, the .second and third may lie ionfiuent, and the fourth 

 is often absent. The bone on each side of the spines is formed by the ankylosed 

 laminae. In the fourth sometimes, but always in the fiftli. the lamina' fail to 

 meet in the middle line, and this leaves a gap. the hiatus sacralis. Th«' median 

 l)orders of this hiatus are ])rolonged downwards as rounded processes, the sacral 

 cornua, to whicli the posterior sacro-coecygeal ligaments are attached. External 

 to the laminae is a second series of small itrominences: these are the articular 



