40 



THE SKELETON 



middle by four transverse ridges [linese transversse] which represent the ossified 

 intervertebral discs and separate the bodies of the five sacral vertebrae. Of the 

 bodies, the first and second are nearly equal in size and are larger than the third, 

 fourth, and fifth, which, in vertical depth, are also nearly equal to each other. At 

 the extremities of the transverse ridges on each side are four openings, called the 

 anterior sacral foramina, which correspond to the intervertebral foramina in other 

 regions of the column, and transmit the anterior divisions of the first four sacral 

 nerves; they are also traversed by branches of the lateral sacral arteries. The fora- 

 mina are separated by wide processes, representing the costal processes of the 

 vertebrae, which unite laterally to form the lateral portion (or mass) [pars lateralis]. 

 The latter is grooved for the sacral nerves, and rough opposite the second, third, 

 and fourth sacral vertebrae, for the origin of the 'piriformis muscle. The lateral 

 part of the fifth sacral vertebra gives insertion to fibres of the coccygeus. 



The dorsal surface is strongly convex and rough. The middle line is occu- 

 pied by four eminences representing the somewhat suppressed spinous processes. 

 Of these the first is the largest, the second and third may be confluent, and the 

 fourth is often absent. The processes are united to form an irregular ridge or 



Fig 



The Sacrum. (Posterior view.) 



Articular process 



Auricular surface 



^^^ ^y 

 Spinous process vi-^ 



Latissimus dorsi 



Articular process 



Transverse process 



Sacral foramen 



Hiatus sacralis leading in 

 to the sacral canal 



Sacral cornu 



Multifidus 

 Sacro-spinalis 



Gluteus maximus 



Notch for fifth sacral nerve 



Apex 



crest [crista sacralis media]. The bone on each side of the spines is slightly 

 hollowed and is formed by the united laminae. In the fourth sometimes, but 

 always in the fifth, the laminae fail to meet in the middle line, leaving a gap 

 [hiatus sacralis] at the termination of the spinal canal, the lateral margins of which 

 are prolonged downward as the sacral cornua. They represent the lower articular 

 processes of the fifth sacral vertebra and give attachment to the posterior sacro- 

 coccygeal ligaments. Lateral to the laminae is a second series of small eminences 

 which represent the articular and mammillary processes of the vertebrae above. 

 The first pair are hirge for the last lumbar vertebra, the second and third are small, 

 and the fourth and fifth arc inconspicuous. Together they form a pair of irregular 

 ridges [cristac sacrales articularesj. 



Immediately lateral to the articular processes are the posterior sacral fora- 

 mina, four on each side; they are smaller than the anterior, and give exit to the 

 posterior primary divisions of the first four sacral nerves. Lateral to the fora- 

 mina on each side are five elevations representing the transverse processes. The 

 first pair, situated at the junction of tlic posterior surface with the base, are large 

 and conspicuous, and serve all for the attachment of ligaments and muscles. 



