THE SKELETAL SYSTEM 



207 



EPISTROPHEUS:-? 



CURVATURES 



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backbone a fairly rigid column. Four curvatures appear in the adult — 

 cervical, thoracic, lumbar, and sacral. 



The Structure of a Vertebra. A typical vertebra consists of a cylin- 

 drical body, the centrum, which is flattened on its superior (cranial) and 

 inferior (caudal) surfaces. A neural arch arises from the dorsal side of 

 the centrum and surrounds a verte- 

 bral canal. That part of the neural 

 arch which connects with the 

 centrum is the pedicle. A spinous 

 process extends backwards and 

 downwards from the mid-dorsal side 

 of the neural arch. That part of 

 the neural arch between the spinous 

 process and the pedicle is the lamina. 

 Anterior and posterior notches or 

 incisures constrict the pedicles so 

 that the incisures of two successive 

 vertebrae form the foramina for the 

 spinal nerves which pass out between 

 the vertebrae. Articular processes 

 or zygapophyses project forwards 

 and backwards from the neural 

 arches. A postzygapophysis of one 

 vertebra overlaps a prezygapophysis 

 of the next vertebra and the two are 

 bound together by ligaments: thus 

 the backbone is strengthened, but 

 at the same time made less flexible. 

 On each side a transverse process 

 projects from the neural arch 

 laterally into the muscles of the body 

 wall. See Fig. 164. 



The Kinds of Vertebra. There 

 are five kinds of vertebrae, cervical, 

 thoracic, lumbar, sacral, and caudal 

 or coccygeal. A distinguishing 

 feature of cervical vertebrae is a 



transverse foramen which in the upper six vertebrae transmits the 

 vertebral artery. The lateral border of this foramen is formed by 

 the fusion of a rudimentary rib with the vertebra. The first two 

 cervical vertebrae are the atlas and the axis or epistropheus. A peculi- 

 arity of the two is that the centrum of the atlas fuses with that of 

 the axis to form the odontoid process upon which the atlas rotates. The 



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Coccygeal. 



Fig. 163. — The human vertebral col- 

 umn viewed from the left side. (Redrawn 

 after Sobotta.) 



