68 THE COMMON FROG. [chap. 



ring (except at the two ends of the backbone) there 

 juts out a bony prominence called a '•' transverse 

 process," and to a certain number of these a bony 

 " rib " is in most vertebrate animals attached (though 

 there are none in the frog), often extending round to 

 join the breast-bone in front, and being capable of 

 more or less motion, so as (by their simultaneous 

 movement) to be able to enlarge oi to contract the 

 cavity of the chest, which they thus enclose and 

 protect. 



That part of each vertebra which is placed next 

 the body cavity is generally the thickest part, and is 

 called the "body," or "centrum." The series of 



Fig. 33.— The Axis Vertebra of Man. c, centrum : .y, neural spine ; d, tubercular 

 process \p, capitular process ; «, anterior articular surface for atlas ; z, post- 

 zygapophysis ; o, odontoid process ; hy, median vertical ndge beneath centrum. 



bodies (or centra) occupy the position which was at 

 fir§t filled by the primitive notochord, the rest of the 

 vertebral rings having been formed in the sides and 

 roof of the canal formed by the upgrowth and union 

 of the two sides of the primitive groove of the 

 embryo. 



The Frog order is distinguished amongst vertebrates 

 as that which has the absolutely smallest number of 

 joints in the backbone. In the frog there are but 

 nine in the front of the coccyx. In the Pipa toad 



