THE SKELETON. 5 1 



Certain ribs present marked features varying from the 

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

 angle. The articular surface of its head is not divided 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 (Cho- 

 loepus). 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 (beginning) of ribs occurs in all 

 regions of the trunk, but they early coalesce with the ver- 

 tebrae except in the thorax. 



PRACTICAL QUESTIONS AND SUGGESTIONS. 



^(i. 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 vertebrae not present on 

 any of the other vertebras? 



/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? 

 p^ 6. Draw cranial 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 cranial aspect of the second lumbar vertebra and 

 label all features. 



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



11. How do you distinguish the caudal from the cranial aspect of 

 any vertebra? 



