274 MAMMALIA. 



proper, are united together by a more or less extensively 

 developed " corpus callosum " or commissure. 



4. The heart consists — as in Birds — of four cavities or 

 chambers, two auricles and two ventricles. The right and 

 left sides of the heart are completely separated from one 

 another, and there is no communication between the pul- 

 monary and systemic circulations. 



5. The cavities of the thorax and abdomen are completely 

 separated from one another by a muscular partition — the 

 diaphragm or midriff. 



6. The respiratory organs are in the form of two lungs 

 placed in the thorax, but none of the bronchi end in air- 

 receptacles, distributed through the body, as in Birds. 



As regards the osteology of the Mammals, the following 

 points should be noticed : — 



With the exception of the Whales and Dolphins {Cetacea), 

 and the Dugongs and Manatees {Sirenia), the vertebral col- 

 umn is divisible into the same regions as in man — namely, 

 into a cervical, dorsal, lumbar, sacral, and caudal or coccy- 

 geal region (see fig. 481). In the Cetacea and Sirenia the 

 dorsal region of the spine is followed by a number of ver- 

 tebrae which compose the hinder extremity of the body, but 

 which cannot be separated into lumbar, sacral, and caudal 

 vertebrae. 



In spite of the great difference which is observable in the 

 length of the neck in different Mammals, the number of 

 vertebrae in the cervical region is extraordinarily constant, 

 being almost invariably seven, as in man. In this respect 

 there is no difference between the Whale and the Giraffe. 

 The only exceptions to this law are the Manatus australis, 

 one of the Sea-cows, which has usually six cervical vertebrse; 

 one of the two-toed Sloths, wiiich has only six ; and the 

 three-toed Sloth {Bradypus tricladylus), which is commonly 

 regarded as possessing nine, though competent anatomists 

 would refer the posterior two of these to the dorsal region. 



The dorsal vertebrae are mostly thirteen in number, but 

 they vary from ten to twenty -four. In Man there are 

 twelve, in one of the Armadillos only ten, and in the two- 

 toed Sloth the maximum is attained. The lumbar vertebrae 



