38 THE FROG CHAP. 



face of the arch, but have the smooth, cartilage-covered 

 surface or facet looking downwards. 



When two vertebrae are placed in position, the convex 

 posterior face of the foremost centrum fits into the con- 

 cave anterior face of its successor, like a cup and ball, 

 and at the same time the posterior articular processes of 

 the first fit over the anterior articular processes of the 

 second. All the touching surface is, as we have seen, 

 capped with cartilage, and the vertebrae can be moved 

 upon one another, either up and down or from side to side. 



The centra and articular processes are the only parts 

 of the vertebrae which are in contact when the bones are 

 in their natural positions. Large gaps or notches are 

 left between the dorsal portions of the arches (see 

 Fig. 8, A) to allow of the requisite amount of up and down 

 movement, and there are similar gaps between the sides 

 of the arches, bounded by the articular processes above 

 and the centra below. These are called the inter- 

 vertebral foramina : through them the nerves pass from 

 the spinal cord. 



The only differences of importance between the vertebrae 

 now under consideration is in the form and direction of their 

 transverse processes, which are specially large in the 

 third, short and devoid of cartilaginous tips in the fifth, 

 sixth, and seventh. 



The first vertebra has no transverse processes, and 

 on either side of the very small " body " its anterior face 

 bears, instead of the ordinary articular processes, a pair of 

 obliquely placed, oval, slightly concave surfaces or facets, 

 covered with cartilage, and serving for the articulation 

 of the condyles of the skull, presently to be described. 

 The transverse processes of the ninth or sacral vertebra 

 are very long and strong, directed obliquely backwards, 

 and tipped with cartilage : to them the arms of the hip 

 girdle are articulated. 



