68 THE HUMAN BODY. 



the other, but having different portions of the complete 

 segment much modified or rudimentary, or even altogether 

 wanting in some regions. Parts which in this sort of way 

 really correspond to one another though they differ in de- 

 tail, which are so to speak different varieties of one thing,, 

 are said in anatomical language to be homologous to one an- 

 other; and when they succeed one another in a row, as the 

 trunk segments do, the homology is spoken of as serial. 



The Cervical Vertebrae. In the cervical region of the 

 vertebral column the bodies of the vertebraa are smaller 

 than in the dorsal, but the arches are larger; the spinous 

 processes are short and often bifid and the transverse pro- 

 cesses appear perforated by a canal, the vertebral foramen. 



The bony bar bounding 

 jp. rt this aperture on the ventral 

 side, however, is in reality 

 a very small rib which has 

 grown into continuity with 

 the body and true transverse 

 process of the vertebra, al- 

 though separate in very early 

 life: the transverse process 

 uiar process. proper bounds the vertebral 



foramen dorsally. In this latter during life runs an artery, 

 which ultimately enters the skull cavity. 



The Atlas and Axis. The first and second cervical ver- 

 tebrae differ considerably from the rest. The first, or atlas 

 (Fig. 19), which carries the head, has a very small body, 

 Aa, and a large neural ring. This ring is subdivided by a 

 cord, the transverse ligament, L, into a dorsal moiety in 

 which the spinal cord lies and a ventral into which the 

 bony process D projects. This is the odontoid process, and 

 arises from the front of the axis or second cervical vertebra 

 (Fig. 20). Around this peg the atlas rotates when the head 

 is turned from side to side, carrying the skull (which ar- 

 ticulates with the large hollow surfaces Fas] with it. 



The odontoid process really represents a large piece of 

 the body of the atlas which in early life separates from its 

 own vertebra and grows on the axis. 



