1 6 THE VERTEBRAL COLUMN. [CHAP. 



The exceptions are : Near the posterior part of the trunk, 

 in nearly all Mammals which possess completely developed 

 hinder limbs, two or more vertebrae become ankylosed 

 together to form the " sacrum," the portion of the vertebral 

 column to which the pelvic girdle is attached. As a rule, 

 none of the other vertebrae are normally united by bone, 

 but in some species there are constant ossific unions of 

 certain vertebrae, more particularly in the region of the 

 neck. These will be specially noticed presently. 



FIG. 2. Anterior surface of human thoracic vertebra^fourth), . c body or centrum ; 

 r neural canal ; / pedicle and / lamina of the arch ; / transverse process ; az 

 anterior zygapophysis. 



Although the vertebrae of different regions of the column 

 of the same animal, or of different animals, present great 

 diversities of form, there is a certain general resemblance 

 among them, or a common plan on which they are con- 

 structed, which is more or less modified by alteration of 

 form or proportions, or by the superaddition or suppression 

 of parts to fit them to fulfil their special purpose in the 

 economy. 



An ordinary vertebra (see Fig. 2) consists in the first place 

 of a solid piece of bone, the body or centrum (c), of the 



