68 THE HUMAN BODY. 



two arches which surround cavities. The dorsal cavity 

 inclosed by the "body" and "neural arch" contained origi- 

 nally part of the spinal cord. The other ring, made up by 



the body of the vertebra dor- 



^^^^11 i^r-^^ sally, the sternum ventrally, 



j ^^ and the ribs on the sides, sur- 



rounds the chest-cavity with 

 its contents. All of these parts 

 together form a typical seg- 

 ment of the axial skeleton, 

 which, however, only attains 

 this completeness in the tho- 

 racic region of the trunk. In 



FIG. 20. Diagrammatic representa- ,, e lrnll if is m-patlv rnnHifiprl 

 tiou of a segment of the axial skeleton, tne SKlll I It IS glCdliy n 



and in the neck and the lower 



P art of the trunk tv ~ ' ibs are 



is the rib-cartilage. either absent or -V. small, 



appearing only as processes of the vertebrae; and the sternal 

 portion is wanting altogether. 



Nevertheless we may regard the whole axial skeleton as 

 made up of a series of such segments placed one in front of 

 another, but having different portions of the complete seg- 

 ment much modified or rudimentary or even altogether 

 wanting in some regions. Parts which in this way really 

 correspond to one another though they differ in detail, which 

 are so to speak different varieties of one thing, are said in 

 anatomical language to be homologous to one another; and 

 when they succeed one another in a row, as the trunk seg- 

 ments do, the homology is spoken of as serial. 



The Cervical Vertebrae. In the cervical region of the 

 vertebral column the bodies of the vertebrae are smaller than 

 in the dorsal, but the arches 



are larger; the spinous pro- BB ' Frt 



cesses are short and often bifid 



and the transverse processes 



appear to be perforated by a 



canal, the vertebral foramen. 



The bony bar bounding this Jf \) \, 



aperture on the ventral side, 



FIG. 21. A cervical vertebra. Frt, 

 however, IS in reality a Very vertebral foramen; Pat, anterior ar- 



small rib which has grown into ticuIar P ro 



continuity with the body and true transverse process of 



