VERTEBEAL COLUMN. 293 



well ; the hindmost vertebrse being reduced to mere 

 rod-like centra. 



B. The Ribs and Sternum. 



These form, with the thoracic vertebrae, the skeletal frame- 

 work of the wall of the thorax, which plays an essential 

 part in the mechanism of respiration, besides protecting the 

 thoracic viscera, and giving origin to some of the extrinsic 

 muscles of the fore-limbs. 



1 . The ribs are curved bony rods, movably articulated with 

 the vertebrse above, and connected at their lower 

 ends with the sternum. There are in the rabbit 

 twelve, or sometimes thirteen, pairs. 



a. A typical rib, such as the fifth, consists of two 



portions of very unequal size, joined together 

 almost at right angles. 

 i. The vertebral portion, which is the larger part 

 of the rib, is a flattened rod of bone, strongly 

 curved above, nearly straight below. The 

 dorsal end or capitulum articulates with a 

 concave surface, furnished partly by the cen- 

 trum of the corresponding vertebra, and partly 

 by that of the vertebra next in front. A short 

 way beyond the capitulum, and on the dorsal 

 surface of the rib, is the tubercle or surface 

 for articulation with the transverse process 

 of the corresponding vertebra ; and imme- 

 diately beyond the tubercle is a short vertical 

 projection for the attachment of ligaments. 



ii. The sternal portion is a short bar of calcified 

 or imperfectly ossified cartilage, connecting 

 the lower end of the vertebral portion with 

 the sternum. 



b. The ribs in general. The ribs increase in length 



from the first, which is very short, to the sixth, 

 behind which they diminish. The first nine pairs 



