OF THE CLASS MAMMALIA. 



247 



pairs ; in the horse, however, there are eighteen, and in the 

 elephant, twenty pairs. The sternum, which in general is 

 flat and smooth, is raised into a crest in the bats, as if to 

 accommodate the greatly increased depressor muscles of the 

 wings, thus adapting the animal for flight, an arrangement 

 we find carried to its highest extent in "birds. In all this 

 class (class mammals) a muscular diaphragm separates the 

 cavity of the chest from the abdomen. 



397. Limbs. In all mammals, with the exception of 

 the cetacea, there are four limbs ; two thoracic, and two abdo- 

 minal or pelvic. In the cetacea there are only two, the 

 thoracic. They are uniformly composed of a basilar portion or 

 base, followed by three principal segments or portions respec- 

 tively, thus : the arm and thigh, the fore-arm and leg, the 

 hand and foot. The basilar portion of the anterior or pectoral 

 extremity is the shoulder blade, or bone called scapula, whose 

 form varies much according to the character of the movements 

 of the animal. In animals which use their pectoral extremi- 

 ties as instruments of prehension as well as support, the 

 scapulae are secured in their place by two bones which rest on 

 the sternum ; these are the collar bones (Fig. 81) ; but in 

 such animals as the horse, using its anterior limbs wholly for 



Fig. 211. 



Fig. 212. 



progression and not prehension, the collar bones are either 

 altogether wanting or reduced to mere vestiges. Some very 

 singular and anomalous animals, also mammals, no doubt, 



