62 



ELEMENTS OF MAMMALIAN ANATOMY 



stricted portion between the capitulum and tubercle; and the 

 shaft, including the portion of the rib l^etwcen the tubercle 

 and its articulation with the sternal or cartilaginous portion. 

 The bend in the shaft beyond the tubercle is the angle. The 

 sternal portion of the rib, consisting of cartilage, is sometimes 



called the costal 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 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 proc- 

 esses of the vertebrae. 



The number of ribs varies from nine 

 ag, Angle; cr, pit for pairs in the cctaccan Hyperoodon to 



articulation with car- ^ . .^jij-u 



tiiaginous portion; h, twcnty-four m the two-toed sloth 



(Cholcepus). 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 de- 

 velopment of Hmbs, the vertebrates have suffered a de- 

 generation 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 vertebrae except in the thorax. 



LABORATORY STUDIES AND SUGGESTIONS 

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

 Describe the differences between the first and last thoracic vertebrae. 



Fig. 30. — Caudal As- 

 pect OF Sixth Rib. 



capitulum; nk, neck; 

 sh, shaft; th, tubercle 

 with facet for articula- 

 tion with transverse 

 process. 



3. What feature common to all thoracic vertebrae not present on any of the 

 other vertebrae? 



