THE EIBS AND STEENUM 273 



pelvis ; and of these the rabbit has only one, or at 

 most two. A certain number of the succeeding ver- 

 tebras, which are fused with one another and with the 

 last true sacral vertebra, are, however, commonly 

 included under the same name, the whole fused mass 

 being spoken of as the sacrum. Reckoned in this 

 way the rabbit has three or more sacral vertebra, the 

 lines of boundary between which can easily be made 

 out, in spite of their fusion. 



a. The first sacral vertebra is much the largest, and is 



produced laterally into a pair of large wing-like 

 expansions which support the pelvis, and are re- 

 garded as modified ribs fused with the vertebra. 

 The neural spine is large and nearly vertical. 



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 

 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 vertebrae above, and connected at their lower 

 ends with the sternum. There are in the rabbit 

 twelve, or sometimes thirteen, pairs. 



T 



