THE \TRTEBRAL COLUMN 163 



processes. Each of the first three of the lumbar vertebrae bears 

 a median ventral projection, the anterior spinous process (processus 

 spinosus anterior), for the attachment of the lumbar portion of 

 the diaphragm. 



Sacral Region 



The sacral vertebrae (vertebrae sacrales) are four in number 

 and are modified for the attachment of the pelvic girdle. In con- 

 trast to the true vertebrae — those united by ligament and articular 

 surfaces — of the remaining portions of the vertebral column, they 

 are false vertebrae, being united in the young by synchondroses, 

 and in the adult coalesced to form a composite structure, the os 

 sacrum (Fig. 83). The axis of the sacrum forms an obtuse angle 

 with that of the lumbar vertebrae, the angle being indicated by a 

 ventral projection, the promontory (promontorium), formed by 

 the last lumbar and first sacral vertebrae where they articulate. 

 The sacrum is the medium through which the vertebral column — 

 in other words, the posterior portion of the trunk— is supported 

 on the posterior limbs. Its anterior dorsal portion bears on either 

 side a roughened area, the auricular surface (facies auricularis), 

 for articulation with the pelvic girdle. This surface is borne for 

 the most part on the transverse process of the first sacral vertebra. 



The sacrum exhibits many features resulting from its formation 

 through the fusion of originally distinct vertebrae. On the ventral 

 or pelvic surface (facies pelvina), the lines of junction may be 

 traced either between the bodies, or between the transverse pro- 

 cesses. Four pairs of apertures on this surface, the anterior sacral 

 foramina (foramina sacralia anteriora), lead into the intervertebral 

 foramina, and give passage to the sacral spinal nerves. On the 

 dorsal surface (facies dorsalis) a pair of posterior sacral foramina 

 in the line of junction of the first and second vertebrae and minute 

 foramina behind the second and third vertebrae transmit the 

 dorsal rami of the first to third sacral nerves. The spinous processes 

 are evident in all four vertebrae.^ The combined articular and 

 mamillary processes are conspicuous only in the first two, but are 

 represented in the succeeding two by low, roughened tubercles. 

 In the middle line dorsally the vertebral arches are separated by 

 conspicuous apertures, the median sacral foramina. 



