THORACIC VERTEBE^ 271 



surfaces for the heads of the first pair of 

 thoracic ribs. 



ii. The neural spines are small, excepting that of 

 the seventh vertebra. 



iii. The transverse processes are perforated at their 

 bases by the vertebrarterial canals : beyond 

 these they divide into dorsal and ventral por- 

 tions, of which the latter, the inferior lamellaB, 

 form prominent bony plates, except in the 

 seventh vertebra, in which they are absent. 



iv. The zygapophyses are borne on the sides of the 

 neural arches ; the anterior facing upwards 

 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 vertebrae 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 i^ 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 



