THE SKELETON. 69 



the vertebra, although separate in very early life: the trans- 

 verse 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 verte- 

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

 (Fig. 22), 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. 23). 

 Around this peg the atlas rotates when the head is turned 

 from side to side, carrying the skull (which articulates 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 becomes united to the axis. 



The Lumbar Vertebrae (Fig. 24) are the largest of all the 

 movable vertebra and have no ribs attached to them. Their 

 spines are short and stout and lie in a more horizontal plane 





FIG. 22. FIG 



FIG. 22. The atlas. FIG. 23. The axis. Aa, body of atlas; D, odontoid process; 

 Fas, facet on frout of atlas with which the skull articulates; and in Fig. 23, ante- 

 rior articular surface of axis; L. transverse ligament; Frt, vertebral foramen; Ap, 

 neural arch; Tp, spinous process. 



than those of the vertebrae in front. The articular and trans- 

 verse processes are also short and stout. 



The Sacrum, which is represented along with the last lum- 

 bar vertebra in Fig. 25, consists in the adult of a single bone; 

 but cross-ridges on its ventral surface indicate the limits of 

 the five separate vertebrae of which it is composed in 

 childhood. It is somewhat triangular in form, its base 



