CHAP. III.] GENERAL CHARACTERS. ii 



The exceptions are, — near the posterior part of the trunk, 

 in nearly all Mammals which possess completely developed 

 hinder limbs, two or more vertebra 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. 



Although the vertebras 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 (<:), of the 

 form of a disk or short cylinder. The bodies of contiguous 

 vertebrae are connected together by a very dense, tough, and 

 elastic fibrous material, called the intei'vcrtebral substance, of 

 peculiar and complex arrangement. This substance forms 

 the main, and in some cases the only, union between the 

 vertebrae. Its elasticity provides for the vertebrae always 

 returning to their normal relation to each other and to the. 

 column generally, when they have been disturbed therefrom 

 by muscular action. 



A process (/) rises on each side from the dorsal surface 

 of the body. These meeting in the middle line above form 

 together an arch, surrounding a space or short canal {nc). 

 As in this space lies the posterior prolongation of the great 

 cerebro-spinal nervous axis, or spinal cord, it is called the 



