BACKBONE 



351 



the rabbit (p. 422) but there are no ribs, these being represented 

 by small knobs of cartilage on the transverse processes. The 

 centra have traces of a canal for the notochord (p. 302). The 

 typical structure is shown by vertebrse two to seven (Fig. 266), 

 in which the centra are procoelous, that is, they are hollow in 

 front and bulged at the back, so that they articulate with one 

 another and the whole series forms a flexible jointed axis. There 



prezyg apophysis 



ntural arch 



neural spine 



postzygepophysis 



prezygap 



cartilaginous 

 Icnob 



transverse process 

 centrum 



postzygap 



pedicle of ^ 

 neural arch 



neural spine 

 trans, process 

 postzygap. 



neural arch 



ganglion of 

 dorsal root 



spinal nerve 



Fig. 266. — The structure of a frog's vertebra. - 



prezygap. 

 neural canal 



spinal cord 

 centrum 



-From Thomson. 



A, Fifth vertebra from above ; B, from the posterior ; C, from the anterior ; D, side view of fifth and sixth 

 vertebrae, not quite articulated, showing position of spinal cord and spinal nerves. An arrow beside 

 each figure points towards the head. 



are certain peculiarities, which enable most of the individual 

 vertebrae to be easily recognised. 



The first has its centrum greatly reduced and there are no 

 transverse processes, so that it is little more than a ring of bone ; 

 it has two hollows in front which fit the occipital condyles of 

 the skull, and a single bulge behind. It is known as the atlas 

 because it carries the skull just as the giant Atlas of Greek legend 

 carried the world on his shoulders. 



The second has its transverse processes directed forwards. 



The third has large transverse processes directly outwards at 

 right angles to the whole backbone. 



