OF THE CLASS MAMMALIA. 233 



vertebras is, with few exceptions, the same in all. The 

 exceptions are the ai, which has nine, and the lamantin, which 

 has six. The giraffe and whale, opposed as they are iu 

 respect of length of neck, have precisely the same number of 

 cervical vertebras. 



396. The conformation of the thorax or chest varies 

 little ; the number of ribs is generally from twelve to fourteen 

 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, a muscular diaphragm separates the cavity of 

 the chest from the abdomen. 



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



Fig. 181 Skeleton of a Cetaceous Animal (the Dugong).* 



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 extre- 

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

 form varies much according to the character of the movement 

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

 ties as instruments of prehension as well as support, the 

 scapulas 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 



* I regret that my friend has placed this figure here, for in point of fact 

 the Dugong is not a cetaceous animal, as I proved long ago. R. K. 



