viii.] GENERAL CHARACTERS. 109 



while in expiration they fall back, and occupy a more 

 oblique position to the axis of the column. 



The sternal ribs are connected with the sternum either by 

 interposed fibrous tissue or by distinct synovial joints. The 

 first is attached to the side of the presternum, the second 

 opposite to the junction of the presternum and the first 

 mesosternal segment, and the succeeding ones opposite to 

 the interspaces between the other mesosternal segments ; 

 though two or more may be attached to the hinder end of 

 the last of these segments. The inferior ends of the so- 

 called " false ribs " are attached by fibrous tissue, or by 

 synovial joints, to the hinder borders of the sternal ribs in 

 front ; though, as before said, the most posterior are free or 

 " floating." 



The ribs of existing Mammals never have "uncinate 

 processes," like those found in Birds and Reptiles. 



The most prevalent number is thirteen pairs ; the lowest 

 is nine (in Hyptroodoii), the highest twenty-four (in the Two- 

 toed Sloth, Cholocpus). 



Special Characters of the Ribs in the various Groups rf 

 Mammalia. 



Order PRIMATES. In man there are normally twelve 

 pairs of ribs, of which the first seven are reckoned as true 

 ribs and the last two as floating ribs. The last pair may 

 be rudimentary or absent ; or, on the other hand, the seventh 

 cervical or the first lumbar vertebra may have an additional 

 movable rib articulated with it. 



The first vertebral rib is much shorter, broader, flatter, 

 and more curved than the others. These gradually increase 

 in length until the seventh, after which they again diminish 

 to the twelfth. In breadth they gradually decrease from the 

 first to the last. 



