THE BONES OF THE TRUNK . iS 



with the twelfth rib, is serially homologous with a lumbar trans- 

 verse process, the superior with a lumbar mammillary process, and 

 the inferior with a lumbar accessory process. The superior 

 articular processes are thoracic in type, whilst the inferior are like 

 those of a lumbar vertebra, being convex and directed outwards 

 and for%vards, or away from each other. Sometimes the superior 

 articular processes are also lumbar in type, being concave and 

 looking inwards and backwards, or towards each other. When 

 this is so, the inferior articular processes of the eleventh thoracic 

 vertebra are also lumbar in type. The twelfth thoracic vertebra 

 very closely resembles a lumbar vertebra, from which, how- 

 ever, it differs in having a facet on the outer surface of each 

 pedicle. 



The thoracic vertebrae receive their blood-supply from the inter- 

 costal arteries. 



The Lumbar Vertebrae. 



The lumbar vertebrae are five in number, and are so named because 

 they occupy the region of the loins. They are the largest of the 

 true vertebrae, and their negative characters are — the absence of 

 a costo- transverse foramen in the transverse process ; and the 

 absence of any kind of costal facet on the side of the body. They 

 increase in size from above downwards, the fifth being the largest, 

 but, as this vertebra has certain distinctive characters, it will be 

 separately described. 



A Typical Lumbar Vertebra. — The body, when viewed from above 

 or below, is reniform, being flattened from above downwards, 

 convex transversely over its antero-lateral surface, and slightly 

 concave transversely on its posterior surface. It is wider from 

 side to side than from before backwards. The anterior depth is 

 slightly greater than the posterior, in adaptation to the forward 

 curve of the vertebral column in the lumbar region. There is no 

 facet on either side of the bod3^ 



The pedicles are short, strong, and directed backwards. The 

 superior vertebral notches are shallow, the inferior being deep and wide. 



The laminae are short, thick, and deep, and their planes are 

 almost vertical. 



The spinous process is axe-shaped, its direction being straight 

 backwards, and it terminates in a round elonga':ed border. 



The articular processes are strong. The superior pair project 

 upwards from the junction of the pedicles and laminae, and the 

 inferior pair project downwards from the lower borders of the 

 laminae. The superior pair are concave, their planes being vertical, 

 and their direction being inwards and backwards, so that thev almost 

 face each other. They stand wide apart, so as to embrace the 

 inferior articular processes of the vertebra above. On the posterior 

 border of each there is a nipple-shaped projection directed back- 

 wards and slightly upwards, called the nuivimillary process {nie'.a- 



