84 OSTEOLOGY. 
attachment of the strong sacro-iliac ligaments. Inferiorly, the edge formed by 
the lateral masses of the fourth and fifth sacral vertebree becomes gradually 
thinner, and at the inferior lateral angle changes its direction and sweeps inwards 
towards the body of the fifth sacral segment. 
The apex, or lower end of the sacrum, 1s formed by the small oval body of the 
fifth sacral vertebra, which articulates with the coccyx. 
The sacral canal follows the curve of the bone; more or less triangular in 
shape above, it becomes appressed and flattened below. Inferiorly, its posterior 
wall is deficient owing to the imperfect ossification of the laminze of the fifth, and, 
it may be, of the fourth sacral segments. Passing obliquely outwards and downwards 
from this canal into the lateral masses on either side are the four pairs of inter- 
vertebral foramina, each of which is connected externally with a V-shaped canal 
which terminates in front and behind in the anterior and posterior sacral foramina. 
The hinder limb of the V is shorter and narrower than the anterior. 
The female sacrum is proportionately broader than the male. Its curves are 
liable to great individual variation, though the absolute depth of the curve is less 
than in the male. 
Variations.—The number of sacral segments may be increased to six or reduced to four (see 
p- 87). Transition forms are occasionally met with in which the first sacral segment displays 
on one side purely sacral characters, @.e. it articulates with the imnominate bone, whilst on the 
opposite side it may present all the features of a lumbar vertebra. Through deficiency in the 
development of the lamine, the neural canal may be exposed throughout its entire length, or 
to a greater extent than is normally the case. (Paterson, Roy. Dublin Soc. Screntific Trans. 
vol. v. Series IT.) 
Coccyx. 
The coccyx consists of four—sometimes five, less frequently three—rudimentary 
vertebrae, which tend to become fused. The first piece is larger than the others; it 
; has an oval hollow facet 
on its upper surface, 
which articulates with 
the body of the last 
sacral segment. Pos- 
teriorly, two processes, 
cornua coccygea, which 
he in series with the 
articular processes of 
the sacrum, extend up- 
wards and unite with 
the sacral cornua, thus 
bridging over the notch 
for the exit of the fifth 
sacral nerve, and con- 
verting it imto a fora- 
. Sacrum. 7. Transverse process. yoen. the last of the 
. Cornu. 8. Transverse process, . : 5 
intervertebral series. 
From the outer side of the body projects a rudimentary transverse process which 
may, or may not, unite with the sacrum close to the lower lateral angle; in the 
latter case a fifth anterior sacral foramen is enclosed. Inferiorly, the body of the 
bone articulates with the succeeding vertebra. The second coccygeal vertebra 
displays slight traces of a transverse process and the rudiments of pedicles. The 
following segments are mere rounded or oval-shaped nodules of bone. 
Fic. 64.—THE Coccyx. 
A. Posterior Surface. B. Anterior Surface. 
1. Transverse process. 3. Sacrum. 
5 
2. Transverse process. 4. Cornu. 6 
Fusion between the lower elements occurs normally in middle life, whilst union between 
the first and second segments occurs somewhat later. It is not unusual, however, to find 
that the first coccygeal vertebra remains separate from the others. Though very variable, as 
a rule, fusion occurs more commonly in the male, and at an earlier age than in the female. 
From the posterior surface of the coccyx the gluteus maximus arises. To its 
external borders are attached the coccygei and levatores ani muscles, and from its 
tip spring the fibres of the sphincter ani. | 
