18 THE VERTEBRAL COLUMN. 



According to the observations of E. Steinbach there are in most cases five coccygeal 

 vertebrae in the male, four or five with about equal frequency in the female. (" Die Zahl der 

 Caudalwirbel beim Menschen," Diss., Berlin, 1889.) 



The vertebral column as a whole. The vertebral column may be regarded as a 

 central axis upon which the other parts of the skeleton are arranged. Superiorly 

 it supports the skull, laterally the ribs, through which also it receives the weight of 

 the upper limbs, and near its lower extremity it rests upon the hip-bones, by which 

 the weight of the body is transmited to the lower limbs. It is a pillar of support to 

 the rest of the skeleton, and protects the spinal cord by enclosing it in a bony 

 canal. Its average length is about 28 inches in the male, 27 inches in the female. 



When seen in profile the column presents four curves, directed alternately for- 

 wards and backwards, forwards in the cervical and lumbar regions, backwards in 

 the dorsal and sacral. The upper curves pass imperceptibly into one another, but 

 at the junction of the last lumbar vertebra with the sacrum a considerable angle is 

 formed, known as the lumbo-mcral or sacro-vertelral angle, causing the promontory 

 to overhang the cavity of the pelvis. The dorsal and sacral curves are primary 

 curves affecting those parts of the column which enter into the formation of the 

 bony-walled cavities, the thorax and pelvis ; they make their appearance at an early 

 period of foetal life, and are due to the conformation of the vertebral bodies : the 

 cervical and lumbar curves are secondary or compensatory curves, necessary to the 

 upright posture, only developed after birth, and dependent mainly on the shape of 

 the intervertebral discs ; in these regions also the principal movements of the spine 

 take place. The curves obviously confer upon the column greater elasticity and 

 security from injury than it would have were it perfectly straight. In the upper 

 dorsal region there is also very frequently a slight degree of lateral curvature, the 

 convexity of which, in most cases, is directed towards the right side, and which is 

 probably connected with the greater use made of the right than of the left arm. 



Viewed from the front, the bodies of the vertebrae are seen to become broader 

 from the axis to the first dorsal, then slightly narrower to the fourth dorsal, and 

 from this vertebra they gradually widen to the base of the sacrum. The width 

 between the extremities of the transverse processes is considerable in the atlas ; small 

 in the axis, it becomes greater as far as the first dorsal vertebra ; thence it is again 

 gradually contracted as far as the last dorsal, and becomes suddenly much greater in 

 the lumbar region. 



In the lateral view, the antero-posterior diameter of the bodies increases in de- 

 scending through the dorsal and lumbar regions. 



Viewed from behind, the spines occupy the middle line. On the sides are the 

 vertebral grooves, corresponding to the larninas, and bounded externally in the cer- 

 vical and dorsal regions by the transverse processes, and in the lumbar by the 

 mamillary processes. Along each groove is a series of spaces between the lamina?, 

 which, in the natural condition, are filled up by the yellow ligaments. The extent 

 of these intervals is very trifling in the neck and in the greater part of the back ; 

 it increases in the lower third of the dorsal, and still more in the lumbar region. 

 The interval between the occipital bone and the arch of the atlas is considerable, 

 and so is that between the last lumbar vertebra and the sacrum. 



The only part of the vertebral column that appears on the surface of the body 

 is the row of spinous processes, and these are subcutaneous from the seventh cervical 

 to the third sacral. The upper cervical spines are deeply placed and can be felt with 

 difficulty in the median interval between the muscular masses of the back of the 

 neck ; the sixth is sometimes long and in such cases may project. The seventh 

 cervical and the following one or two dorsal spines are prominent ; the others lie at 

 the bottom of the long spinal furrow produced by the eminence of the spinal muscles 

 on each side. 



