152 THE VERTEBRATE SKELETON. 



dicular portion consisting of the skeleton of the limbs and 

 their girdles. 



1. THE AXIAL SKELETON. 



A. THE VERTEBRAL COLUMN. 



The vertebral column is a tube, formed of a series of ten 

 bones which surround and protect the spinal cord. Of these 

 ten bones nine are vertebrae, while the tenth is a straight rod, 

 the urostyle, and is almost as long as all the vertebrae put 

 together. The second to eighth vertebrae inclusive have a very 

 similar structure, but the first and ninth differ from the others. 



Any one of the second to eighth vertebrae forms a bony 

 ring with a somewhat thickened floor, the centrum or body, 

 which articulates with the centra of the immediately preceding 

 and succeeding vertebrae. The articulating surfaces are 

 covered with cartilage and are procoelous, or convex in front 

 and concave behind. The eighth vertebra is however amphi- 

 coelous or biconcave. The centrum of each vertebra encloses 

 an isolated vestige of the notochord. The neural arch forms 

 the roof and sides of the neural canal, which is very spacious 

 in the anterior vertebrae, but becomes more depressed in the 

 posterior ones. The arch bears the neural spine, a low 

 median ridge of variable character, and is drawn out in 

 front and behind, forming the two pairs of articulating 

 surfaces or zygapophyses by means of which the vertebrae 

 are attached together. Of these the anterior articulating 

 surfaces or prezygapophyses look upwards and slightly 

 inwards, while the posterior articulating surfaces or post- 

 zygapophyses look downwards and slightly outwards. The 

 sides of the neural arches are drawn out into a pair of 

 prominent transverse processes. Those of the second 

 vertebra look somewhat forwards, those of the third look 

 directly outwards or somewhat forwards, while those of 

 the fourth, fifth, and sixth are directed slightly backwards, 

 and those of the seventh and eighth nearly straight outwards. 



