THOEACIC VEKTEBEJE 263 



and inwards, the posterior downwards and 



slightly outwards. 

 3. The thoracic vertebrae. 



These are usually twelve, sometimes thirteen, in 

 number, and are characterised by bearing movably 

 articulated ribs. The neurapophyses are long, 

 especially in front : that of the tenth or anticlinal 

 vertebra is nearly vertical, those in front of it sloping 

 backwards, and those behind forwards. The pos- 

 terior two or three thoracic vertebras are larger than 

 the others, and gradually approach the lumbar 

 vertebrae in size and shape. 



a. A typical thoracic vertebra : any one of the first 



six. 



i. The centrum is short, and thick from above 

 downwards. On each side in front is a small 

 facet, which, with a similar one on the hinder 

 edge of the vertebra next in front, forms the 

 articular surface for the head of the rib. At 

 the posterior end are a similar pair of facets 

 for the ribs of the succeeding vertebra. 



ii. The neural arch is notched, both before and 

 behind, for the passage of the spinal nerves. 



iii. The transverse processes are short, stout, and 

 horizontal, and bear on their under surfaces 

 the articular facets for the tubercles of the ribs. 



iv. The neurapophysis is long and slender, and 

 slopes obliquely backwards. 



v. The zygapophyses. The pre-zygapophyses face 

 upwards and outwards, the post-zygapophyses 

 downwards and inwards. 



b. The posterior three or four thoracic vertebrae. 



i. The centra are larger than in the anterior 

 thoracic vertebrae : the facet for the head of 

 each rib is confined to the anterior end of the 

 centrum, the posterior end having no articular 

 surface. 



