56 



INTRODUCTION. 



clearly, it will be best to examine, singly, one of the bones of which it 

 consists. Each vertebra, then, has a main portion, termed its body, 

 usually of a compressed circular, or oval form, having an anterior 

 and posterior flattened, or slightly concave, surface, by which it is 

 united to the body of the one preceding it, and also to the body of the 

 next in rotation. Sometimes, however, the body is elongated (as in the 

 cervical vertebrae of the Giraffe), and, instead of having a flat anterior and 

 posterior articulating surface, has a convex articulating surface ante- 

 riorly, and a concave one posteriorly, in order to effect a ball and 

 socket mode of articulation with the one preceding and the one succeed- 

 ing it. To the body a (fig. 38), certain processes, or projections, of con- 

 siderable magnitude, are appended, which bound, posteriorly, a large 

 38 aperture, forming, when the vertebral column is not taken 



to pieces, a canal for the reception of the spinal chord. 

 Of these processes, one, 5, is termed the spinous ; four 

 are termed the oblique, c, c, c, c ; and two the transverse, 

 d, d : a, represents one of the articulating surfaces of the 

 body of the vertebra. When in their proper situation, 

 the flattened bodies being in apposition, the two lower 

 and two upper of the oblique processes of each adjoining 

 vertebra are connected together, the lower of one ver- 

 one of the vertebrae, tebra, being overlaid by the upper of the vertebra next 

 succeeding. The bodies of the vertebrae, it must be observed, are not 

 in immediate contact ; for between them is interposed a substance of 

 considerable thickness, of a cartilaginous nature, and highly elastic, to which 

 the flexibility of the spine of the Mammalia is entirely owing. Were the 

 bones formed into a column, by being simply fitted together, without this 

 intervening cushion, the whole would be rigid, and incapable of those varied 

 movements which it exhibits. This plan of allowing motion, by the 

 interposition of thick elastic cushions, is, in the vertebral column, attended 

 with one singular and most important advantage ; viz., that while the whole 

 column may be bent forward, backward, or laterally, into arches of 

 various degrees of flexion, each separate bone has, in itself, but a very 

 limited sphere of mobility, whence it is impossible for the relative position 

 of any two to be so abruptly and decidedly altered as to produce an acute 

 angle in the column a mode of flexure, which could not take place 

 without injury to the medulla spinalis enclosed in the canal, which has 

 been already spoken of. 



The true vertebral column is divided into several portions : the cer- 

 vical, or that of the neck ; the dorsal, or that of the back ; the lumbar, 

 or that of the loins ; and the sacral, which is consolidated, and joins the 

 pelvis : by these appellations are designated the separate vertebrae of 

 each of these divisions. 



