92 THE RIBS. [chap. 



least so in the Pcrissodactyla. The anterior ribs have 

 scarcely any curve, the thorax being very narrow in this 

 region. The sternal ribs (see Fig. 35, p. 78), especially 

 those near the front of the series, are short, stout, rather 

 flattened or prismatic, tolerably well ossified, and articulated 

 with the vertebral ribs by a cup-and-ball synovial joint. 

 The Artiodactyles have from 12 to 15 pairs of ribs, the 

 Horse and Tapir 18, the Rhinoceros 19, the Elephant 19 

 or 20, and the Hyrax 20 to 22. 



In the Sirenia, the total number of ribs is very great, 

 though but few are attached to the sternum. In the 

 Manati they acquire an extraordinary thickness and so- 

 lidity of texture. This animal has seventeen pairs, of 

 which but three are attached by flexible cartilages to the 

 sternum. 



Order Cetacea. — In the Whalebone Whales the ribs differ 

 greatly from those of the rest of the Mammalia in their ex- 

 tremely loose connection, both with the vertebral column 

 above and with the sternum below, probably to allow of 

 greater alteration in the capacity of the thorax in respiration, 

 necessitated by the prolonged immersion beneath the sur- 

 face of the water which these animals undergo. 



At their vertebral extremities they are attached only by 

 their tubercle to near the end of the transverse process, but 

 apparently not by synovial articulation. The heads of only 

 a few of the anterior ribs are developed, and are rarely suf- 

 ficiently long to reach the bodies of the vertebras, their 

 place being supplied by a ligamentous band. The first rib 

 is the only one connected with the sternum, either directly 

 or indirectly, the whole of the remainder being free or 

 floating ribs. The sternal ribs are mere cartilaginous rudi- 

 ments, connected by an intermediate layer of fibrous tissue 

 to the inferior extremity of the vertebral rib ; at least such 



