72 



Anatomy oi? the Rabbit. 



more nearly toward, or away from, the median plane, instead of to the 

 dorsal or ventral surface. The anterior articular surfaces are borne on 

 the bases of stout, upwardly-directed mammillary processes (processus 

 mammillares). The latter are most characteristic of the lumbar verte- 

 brae, but may be seen to arise in the posterior thoracic region as small 

 elevations of the transverse 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. 



The sacral vertebrae (vertebrae sacrales) are four in number. In 

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

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

 false vertebrae, united in the young by synchondroses, and in the adult 

 coalesced to form a composite structure, the os sacrum (Fig. 28). The 



p.a.s 



B 



c.v. P'?s- P-' 



Fig. 28 The os sacrum: A, ventral (pelvic) surface; B, dorsal surface; 

 C.V., bodies of coalesced vertebrae; fa., auricular surface; f.s.a., anterior 

 sacral foramina; f.s.m., median sacral foramina; f.s.p., posterior sacral 

 foramina; p.a.s., superior articular process of first vertebra; p.m., mam- 

 millary process of first vertebra; pr. promontory; p.s., spinous processes. 



axis of the sacrum forms an obtuse angle with that of the lumbar ver- 

 tebrae, the angle being indicated by a ventral projection, the promon- 

 tory (promontorium), formed by the last lumbar and first sacral ver- 

 tebrae. 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. 



