38 THE FROG CHAP. 



When two vertebrae are placed in position, the convex 

 posterior face of the foremost centrum fits into the concave 

 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 vertebras 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 intervertebral 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 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 backwards, and tipped with 

 cartilage : to them the arms of the pelvic girdle are articu- 

 lated. 



The eighth vertebra differs from its predecessors in having its body 

 concave behind as well as in front. Corresponding with this, the ninth 



