THE SKELETON. oe 
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 vertebre. 
The number of ribs varies from nine pairs in the cetacean 
Hyperoddon to twenty-four in the two-toed sloth (Cho- 
lepus). 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- 
tebrz 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- 
tebre. 
3. What feature common to all thoracic vertebra 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 vertebra? 
6. Draw cranial aspect of atlas and label all features. 
7. Compare the third and seventh cervical vertebre. 
8. What two features serve to distinguish the lumbar from all 
other vertebre? 
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? 
