66 ELEMENTS OF MAMMALIAN ANATOMY. 



tal rib. In the sixth rib the costal portion is about half as 

 long as the vertebral portion. 



Certain ribs present marked features varying from the 

 sixth. The first rib is stout and flat and has no distinct 

 ande. The articular surface of its head is not divided 



O 



into two facets, as is the case in all the others except 

 the three caudal ones. The lengths of the ribs increase 

 from the first to the ninth. The last three ribs have no 

 necks and no tubercles for articulation with the transverse 

 processes of the vertebrae. 



The number of ribs varies from nine pairs in the 

 cetacean Hyperoodon to twenty-four in the two-toed 

 sloth (Choloepus). In most fishes and snakes, ribs are 

 present throughout both the trunk and tail regions, but 

 with the assumption of life on land, and the development 

 of limbs, the vertebrates have suffered a degeneration of 

 the ribs in all parts except the middle portion of the 

 trunk. In the embryo, however, anlage of ribs occurs 

 in all regions of the trunk, but they early coalesce with 

 the vertebrae except in the thorax. 



PRACTICAL QUESTIONS AND SUGGESTIONS. 



1. Draw caudal aspect of third thoracic vertebra and label all features. 



2. Describe the differences between the first and last thoracic ver- 

 tebrae. 



3. What feature common to all thoracic vertebras not present on any 

 of the other vertebrae? 



4. Draw the caudal aspect of the fourth cervical vertebra. 



5. What feature common to all cervical vertebrae except seventh 

 but not present in any other vertebrae? 



6. Draw cephalic aspect of atlas and label all features. 



7. Compare the third and seventh cervical vertebrae. 



8. What two features serve to distinguish the lumbar from all other 

 vertebrae? 



9. Draw the cephalic aspect of the second lumbar vertebra and label 

 all features. 



10. Wherein does the seventh lumbar vertebra differ from the first? 



