THE FALSE OR FIXED VERTEBR-. 81 
superior articular processes are stout, oval, curved plates of bone, fused in front with 
the pedicles and lamine, and having their concave articular surfaces vertical and 
in-turned. Externally, and on their posterior edge, the bone rises in the form of an 
elongated oval tubercle, the mammillary process (processus mamunillaris); these are 
in correspondence with the superior tubercles of the lower thoracic transverse pro- 
cesses. 
The inferior articular processes lie on either side of the root of the spinous 
process supported on the inferior margin of the lamine. Their articular surfaces, 
oval in outline, convex from side to ‘side, and plane from above downwards, are 
out-turned. The inferior articular processes are much closer together than the 
superior ; so that when the vertebre are articulated the superior articular processes 
of the lower vertebra embrace the inferior articular processes of the higher 
vertebra. 
The fifth lumbar vertebra is characterised by the size of its body, which is 
the largest of all the vertebre. Further, the under surface of the body is cut 
away at the expense of its posterior part: hence the thickness of the centrum in 
front much exceeds that of the vertical diameter behind. The transverse process 
is pyramidal in form, and stouter than those of the other lumbar vertebrae. It 
arises by a broad base from the side of the back of the body, as well as from the 
pedicle, and is directed outwards and a little backwards and upwards. Its upper 
surface is slightly grooved by the superior intervertebral notch. A deep notch 
. separates it posteriorly from the superior articular processes, which are less 
in-turned than in the other members of the series, their articular surfaces being 
directed more backwards than inwards, and displaying less concavity. The inferior 
articular processes are further apart than is the case with the other members of 
the series, they he in line with the superior. The spinous process is shorter and 
narrower than the other lumbar spines, particularly so in the female. 
Variations.—The mammillary and accessory processes are sometimes unduly developed. The 
neural arch of the fifth lumbar vertebra is occasionally interrupted on either side by a synchon- 
drosis which runs between the upper and lower articular processes. In macerated specimens the 
two parts of the bone are thus separate and independent. The anterior includes the centrum, to- 
gether with the pedicles, transverse and superior articular processes ; the posterior comprises the 
inferior articular processes, the laminee, and the spine-—Turner (Challenger Reports, vol. xvi.). 
TEE PALSE OR FIXED VERTEBRA: 
THE SACRUM. 
The sacrum (os sacrum), of roughly triangular shape, is formed by the fusion, 
normally, of five vertebree. The anterior surface of the bone is slightly hollow from 
_ side to side and concave from above downwards, the curve being usually most 
pronounced opposite the third sacral segment. The central part corresponds to 
the bodies of the sacral vertebrie, the lines of fusion of which are indicated by a 
series of four parallel ridges which cross the central part of the bone at gradually 
diminishing intervals from above downwards; externally, these ridges disappear 
on either side on the inner walls of the four anterior sacral foramina (foramina 
sacralia anteriora). The size of these holes decreases from above downwards. The 
upper and under border of each foramen is formed by a stout bar of bone, of which 
there are five on each side, corresponding in number with the vertebra present. 
These unite externally so as to form the lateral mass (pars lateralis), and thus 
enclose the foramina to the outer side, though here the edge is not abrupt, but 
sloped so as to pass gradually into the canal. The large anterior divisions of the 
sacral nerves pass through these foramina and occupy the shallow grooves. The 
bone is broadest across the first sacral vertebra, tends to narrow opposite the 
second, and again usually increases in width opposite the third. When this 
condition is well marked, the edge has a notched appearance (sacral notch) which 
assists in the interlocking of the sacro-iliac joint. The surface of bone between 
and external to the first, second, third. and fourth foramina affords attachment to 
the fibres of origin of the pyriformis, whilst on the edge external to and below the 
fourth foramen the coccygeus is inserted. 
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