STRUCTURE OF VERTEBRA. 395 



present and more variable in their shape. They 

 form articular surfaces for the purpose of being 

 connected with the surfaces of corresponding 

 processes in the contiguous vertebra. Of these 

 there are four (a, a, a, a) belonging to each ver- 

 tebra, two in front, and two behind. These, 

 however, should not be included among the 

 primary elements of the vertebrae, because we 

 find them, in different instances, occupying 

 different positions, and formed sometimes by 

 extensions of the bodies, and at other times of 

 the leaves. In following them through the se- 

 veral tribes of animals, we observe them shifting 

 their places, in various ways, and not even pre- 

 serving any constancy in their number. They 

 are wholly absent in fishes : in the crocodile, 

 and other reptiles, they approximate so as to 

 form three articular surfaces, namely, two close 

 to one another, and a third posterior to these. 

 In the Ornithorhyncus, while the latter retains 

 its situation in the middle, the other surfaces 

 have separated from each other, and have tra- 

 velled outwards, taking their stations upon the 

 leaves. In the Mammalia, the middle surface 

 has wholly disappeared, and the outer surfaces 

 have risen into what are termed the oblique pro- 

 cesses. 



In addition to these, accessory bones are often 

 developed to suit particular occasions. Thus in 

 fishes, we see that one or two additional pieces 



