THE RIBS AND STERNUM 265 



b, The hinder sacral vertebrae are much smaller, and 

 decrease in size from before backwards. Their 

 number is variable, increasing with age owing to 

 fusion with the anterior caudal vertebrae. 



The anterior outer border of the second sacral 

 vertebra just touches the pelvis, but it is doubtful 

 whether any true costal element is present in it. 



6. The caudal vertebrae. 



These are about sixteen in number : they decrease 

 in size from before backwards, gradually losing all 

 their processes, and ultimately the neural arches as 

 well, the hindmost vertebrae being reduced to mere 

 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 the extrinsic muscles of 

 the fore-limbs. 



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

 the vertebrae 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 t consists of two por- 

 tions of very unequal size, connected 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 



