THE SPINAL COLUMN. 



The spinal column is the central part of the skeleton. It supports the head, 

 bears the ribs, thus indirectly supporting the arms, and encloses the spinal cord. It 

 gives origin to many muscles, some passing between different parts of the spine, 

 others connecting it with the body. These purposes demand great strength and 

 flexibility. The spine is composed of* many pieces united by tough fibro-cartilagi- 

 nous disks, by which the force of shocks is broken and the great range of move- 

 ment is distributed among many joints. It is convex behind in the regions of the 

 thorax and pelvis, so as to enlarge those cavities, and has forward convexities in the 

 neck and loins. The numerous prominences which it presents serve for the support 

 of the ribs, the attachment of muscles, and the interlocking of the various pieces. 

 The spinal column is firmly fixed near the lower end between the bones of the pelvis. 



The bones composing this column are called vertebrcs, of which in the adult 

 there are thirty-three or thirty-four in all. They are divided into five groups. The 



Fig. 139. 



Spinous process 



Facet for tubercle 

 of rib 



Transverse process 

 Superior articular process 



Demi-facet for head of rib 



Sixth thoracic vertebra from above. 



first seven are the cervical ; the next twelve, which bear ribs, are the thoracic ; the 

 next five are the lumbar, making twenty-four above the pelvis. These are known 

 as the presacral vertebrae. The remainder are in the adult united into two bones, 

 the first five forming the sacrum, the last four or five the coccyx. As many as 

 thirty-eight are seen in the young embryo, but some disappear or are fused. 



With the exception of the first two, the atlas and the axis, which require a 

 separate description (page 119), the vertebrae above the sacrum present the following 

 features, which are common to all, but which are modified in the different regions : 

 (i) a body^ or centrum ; (2) o. pedicle'^ springing from the back of the body on either 

 side, supporting (3) the lami7ia,^ a plate which meets its fellow in the middle line to 

 form an arch bounding the spinal or vertebral foramen^ for the spinal cord. Each 

 vertebra gives origin to several processes. — namely, (4) a spinous process,^ springing 

 from the point of union of the larrinae ; (5) a transverse process on each side, pro- 

 jecting outward from the junction of the pedicle and lamina ; (6) two articulating 



' Corpus. 



114 



' Radix arcus vertebrae. ^ Arcus. * Foramen vertebrate. ^ Processus spinosus.^ Processus transversus. 



