26 AN INTRODUCTION TO ZOOLOGY 



from the junctions of the pedicles and laminae. The front or pre- 

 zygapophyses have smooth cartilage-covered surfaces, the articular 

 facets, facing upwards and a little inwards, while the post-zygapo- 

 physes have similar facets directed downwards and outwards. 

 Thus, in addition to the intercentral joint or articulation, a pair of 

 zygapophyseal articulations is present between successive vertebrae. 

 It is characteristic of the vertebrates in general that when zygapo- 

 physes are present articulating surfaces of the anterior face upwards 

 or inwards, or both, and the posterior downwards or outwards, or 

 both. 



The first vertebra, or atlas, differs markedly from the rest. The 

 centrum and neural spine are reduced, and the transverse processes 

 and pre-zygapophyses are entirely absent. Its anterior end is 

 provided with two concavities, by means of which it articulates 

 with the skull. The third vertebra has very well-developed trans- 

 verse processes, serving for the attachment of muscles connected 

 with the girdle bearing the fore-limbs. The centrum of the eighth 

 vertebra is hollow in front and behind, a condition that we describe 

 as amphicoelous. The ninth vertebra is also modified, and since its 

 long somewhat backwardly directed transverse processes articulate 

 with the girdle of the hind limbs, it is called the sacrum. The front 

 end of its centrum is convex, the posterior end bears two small 

 knobs and has no zygapophyses. 



Apart from the differences in detail, the nine vertebrae form a 

 series of similar structures repeated one behind the other. Structures 

 that are alike, made of the same parts, and developed in the same 

 way, are said to be homologous, and when repeated, as in the 

 vertebral column, are described as serially homologous. When 

 placed side by side, as in the living frog, a gap is left between the 

 pedicle of one vertebra and the next. This is the intervertebral 

 foramen, and through it the nerves from the spinal cord pass into 

 the body. 



The urostyle is a long unsegmented rod of bone, bearing 

 at its anterior end two facets, which articulate with the knob-like 

 projections from the centrum of the sacrum. A short distance 

 behind these is a pair of small foramina, to allow the last pair of 

 spinal nerves to leave the neural canal. The urostyle is to be 

 regarded as representing a series of fused and reduced vertebrae, and 

 faint indications of segmentation, or even traces of one or two 

 vertebrae, are sometimes to be seen at the anterior end. 



The frog is markedly different from ourselves in not possess- 

 ing any ribs nor any indications of them. 



