VERTEBRAL COLUMN AS A WHOLE. 87 
As viewed from the side, the bodies display a gradual increase in their antero- 
posterior width until the second lumbar vertebra is reached, below which this 
diameter is slightly reduced. In the sacral region the reduction in this diameter is 
great in the first and second sacral segments, more gradual and less marked in the 
last three segments. 
The facets for the heads of the ribs in the upper thoracic region he on the sides 
of the bodies; those for the tenth, eleventh, and twelfth are placed further back on 
the pedicles. 
The intervertebral foramina increase in size from above downwards on the 
movable part of the column, being largest in the Jumbar region. In the sacral 
region they decrease in size from above downwards. In the cervical region the 
two highest cervical nerves pass out behind the articular processes of the atlas and 
axis, and lie, therefore, behind the corresponding transverse processes of these 
vertebre. The succeeding cervical nerves pass out through the intervertebral 
foramina which are placed between the transverse processes and in front of the 
articular processes. In the thoracic and lumbar vertebree the intervertebral 
foramina lie in front of both the articular and transverse processes. The arrange- 
ment of these foramina in the sacrum has been already sufficiently explained. 
The neural canal for the lodgment of the spinal cord and its meninges is largest 
in the cervical and lumbar regions, in both of which it assumes a triangular form ; 
whilst it is narrow and circular in the thoracic region. These facts are correlated 
with the movements of the column which are most free in those regions where the 
canal is largest, 7.e. the neck and loins. 
The average length of the vertebral column is from 70 to 73 centimetres, or from 
274 to 283 inches. Of this the cervical part measures from 13 to 14 cm.; the thoracic, 
27 to 29 em.; lumbar, 17 to 18 em. ; and the sacro-coccygeal, 12 to 15 cm. The individual 
differences in the length of the column are less than one might expect, the variation in 
height of different individuals being often largely dependent on the length of the lower 
limbs. In the female the average length of the column is about 60 centimetres, or 234 
inches, and the curve in the lumbar region is usually more pronounced. 
Architecture.—The vertebree are formed of spongy bone confined within a thin and dense 
envelope. In the bodies the arrangement of the cancellous tissue, which is traversed by venous 
channels, is such as to display a vertical striation with lamellz arranged horizontally. The 
external, superior, and inferior walls are very thin—that directed to the neural canal being 
usually thicker and denser than the others. In the pedicles and roots of the transverse processes 
the cancellous tissue is much more open. The outer envelope is much thicker where it bounds the 
neural ring, and where it forms the bottom of the superior and inferior intervertebral notches. In 
the laminz the spongy tissue is confined between two compact layers, of which that directed to the 
spinal canal is the thicker. In the spinous processes the upper edge is always the more compact. 
Variations.—Numerical Variations of the Column as a Whole.—TIncrease in the number of 
vertebral segments is usually due to differences in the number of the coecygeal vertebrae ; these 
may vary from four—which may be regarded as the normal number—to six. The number of 
presacral or movable vertebra is normally 24 (7 C, 12 D, and 5 L). This number may be in- 
creased by the intercalation of a segment either in the thoracic or lumbar region without any 
alteration in the number of the sacral or coccygeal elements: thus we may have 7 C, 13 D, and 
5 L, or 7 C, 12 D, and 6 L, or may be reduced by the disappearance of a vertebral segment—thus, 
70,12 D,and4L. Such an arrangement presupposes developmental errors either of excess or 
default in the segmentation of the column. On the other hand, the total number of vertebral 
segments remaining the same (24 or 25), we may have variations in the number of those assigned 
to different regions due to the addition of a vertebral segment to one, and its consequent sub- 
traction from another region. Thus, in the 24 presacral vertebrze, in cases of the occurrence of 
cervical ribs the formula is rearranged thus—6 C, 13 D, and 5 L, or, in the case of a thirteenth 
rib being present, the formula would be 7 C, 13 D, 4 L, as happens normally in the gorilla and 
chimpanzee. Similarly, the number of the presacral vertebra (24) may be increased by the 
withdrawal of a segment from the sacral region—7 C, 12 D, 6 L, and 4 S—or diminished by an 
increase in the number of the sacral vertebrze, as in the formula 7 C, 12 D,4 L,and 68. In- 
erease in the number of sacral segments may be due to fusion with a lumbar vertebra, or by the 
addition of a coccygeal element: the latter is more frequently the case. This variability in the 
constitution of the sacrum is necessarily correlated with a shifting backwards and forwards of 
the pelvic girdle along the vertebral column. Rosenburg considers that the 26th, 27th, and 28th 
vertebree are the primitive sacral segments, and that the sacral characters of the 25th vertebrae 
(the first sacral segment in the normal adult column) are only secondarily acquired. He thus 
supposes that during development there is a forward shifting of the sacrum and pelvic girdle, 
with a consequent reduction in the length of the presacral portion of the column. This view is 
opposed by Paterson (Roy. Dublin Soc. Scientific Trans. vol. v. Ser. I1.), who found that ossification 
took place in the ale of the 25th vertebra (first adult sacral segment) before it made its appear- 
