THE SACRUM 41 



the tooth (or odontoid process) and is held in its place in the front part of 

 the ring of the atlas (Fig. 33) by a strong ligament, which prevents it from 

 pressing upon the spinal cord. 



The thoracic vertebras are peculiar, in that their bodies present 

 marks for the heads of ribs; also, they have long transverse and 

 spinous processes. 



FIG. 35. A THORACIC VERTEBRA, SHOWING MARKS FOR HEAD OF 'Rrs. (Morris.) 



The lumbar vertebrae are the largest and strongest in the 

 column, the bodies being conspicuously thicker than in the other 

 regions, especially in the case of the fifth. 



There are various other modifications of bones in the three regions 

 cervical, dorsal, and lumbar which need not be mentioned here. 



FIG. 36. A LUMBAR VERTEBRA IN SECTION TO SHOW THE PRESSURE CURVES. 



(Morris.) 



Sacrum. An irregular bone formed by the consolidation of 

 five incomplete vertebrae, and joined to the last lumbar. Its 

 general shape is that of a curved wedge; it is placed with the 

 base upward, and the concavity forward, forming the "hollow of 

 the sacrum." A canal extends from the base to the apex, called 

 the sacral canal, which is a continuation of the spinal (or neural) 

 canal. 



There are two sets of short canals, running from front to back 

 through the sacrum. Seen from the front they present, the an- 



