124 ANATOMY OF THE RABBIT. 
region as small elevations of the transverse processes. Each of the 
first three of the lumbar vertebrae bears a median ventral pro- 
jection, 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, uvited in the young by synchon- 
droses, and in the adult coalesced to form a composite structure, the 
os sacrum (Fig. 57). 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. 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 
lie in the line of junction of the first and second vertebrae. 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, rough- 
ened tubercles. In the middle line dorsally the vertebral arches 
are separated by conspicuous apertures, the median sacral fora- 
mina. 
The caudal or coccygeal vertebrae (vertebrae caudales, 
s. coccygeae) are sixteen in number. There are segments of small 
size, increasing slightly to the third, and then gradually decreasing 
to the end of the column. The arches are complete in the first 
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