38 A MANUAL OF ANATOMY 



Secondary Centres. — Each vertebra has two secondary centres for the 

 epiphysial plates — one on the upper surface, and the other on the under 

 surface of the rudimentary vertebra (centrum or body). In addition, the 

 lirst coccygeal vertebra has two special secondary centres, one for each comu. 



Union of the four coccygeal vertebrae takes place from below upwards, as 

 in the sacrum. It commences shortly after the eighteenth year, and is not 

 completed until the thirtieth year or later, the last two coccygeal vertebrae to 

 join being the first and second. In advanced life it is not uncommon to find 

 the coccyx snkylosed to the sacrum, thus forming one sacro-coccygeal bone. 



The Vertebral Column as a Whole. 



The vertebral column supports the other parts of the skeleton, 

 directly or indirectly. Its average length is about 28 inches in 

 the male, and rather less in the female. When viewed from the 

 front, it presents four pyramids. The first extends from the axis 

 to the seventh cervical vertebra, its base being dovi^nwards. The 

 second extends from the first to the fourth thoracic vertebra, its 

 base being upwards. The third extends from the fifth thoracic to the 

 fifth lumbar, its base being downwards. The fourth extends from 

 the base of the sacrum to the tip of the coccyx. These pyramids 

 are due to the differences in breadth of the bodies in different parts. 



The column presents certain curves, which are arranged in two 

 groups, antero-posterior and lateral. The antero-posterior group 

 comprises four curves, named cervical, thoracic, lumbar, and 

 pelvic. The cervical and lumbar curves have their convexities 

 directed forwards, and the thoracic and pelvic curves have their 

 convexities directed backwards. The lumbar and pelvic curves 

 meet rather abruptly and form a projection, called the sacro- 

 vertehral angle, which is estimated at 117 degrees in the male and 

 130 degrees in the female. The curves impart springiness or 

 elasticity to the column, and so guard it against shock. The 

 thoracic and pelvic curves are associated with the thoracic and 

 pelvic cavities, the capacity of which they serve to increase. They 

 appear in early foetal life, and are known as primary curves. The 

 cervical and lumbar curves do not appear until after birth, and 

 are known as secondary or compensatory curves. The primary 

 curves are brought about by the greater depth posteriorly of the 

 thoracic and sacral bodies, whilst the compensatory curves are 

 largely due to the intervertebral discs, though in the lumbar region 

 the greater depth of the bodies anteriorly, especially in the fifth 

 lumbar, must also be taken into account. 



The lateral group comprises two curves. One is situated in the 

 upfper thoracic region, with its convexity directed towards the right 

 side in right-handed persons, and it is to be regarded as due to the 

 greater use made of the right arm. To compensate for this curve 

 there is another slight curve in the upper lumbar region, with the 

 convexity to the left. 



When viewed anteriorly, the column presents the bodies, which 

 form the pyramids already described. When viewed laterally, 

 it presents the sides of the bodies, pedicles, intervertebral foramina, 

 and articular and transverse processes. The intervertebral foramina 



