GENERAL CHARACTERS OF THE MAMMALIA. 275 



are usually six or seven in number, rarely fewer than four. 

 In Man they are five in number, and. they are reduced to 

 two in the two-toed Sloth, one of the Ant-eaters, and the 

 Duck-mole. 



The first vertebra, or atlas, always bears two articular 

 cavities for the reception of the two condyles of the occi- 

 pital bone, and the second vertebra, or axis, usually has 

 an " odontoid " process on which the head rotates. In 

 the true Whales, however, in which the cervical vertebras 

 are anchylosed together to a greater or less extent, 

 and the neck is immovable, the odontoid process is also 

 wanting. 



The number of lumbar and sacral vertebra, as we have seen, 

 varies in different Mammals ; but ordinarily some of the ver- 

 tebrse are anchylosed into a single bone, and have the iliac 

 bones abutting against them, thus constituting the " sacrum " 

 of human anatomists. In the Cetacea and Sirenia, in which 

 the hind-Kmbs are wanting, and the pelvis rudimentary, 

 there is no " sacrum." 



Tlie thoracic cavity or chest in Mammals is always en- 

 closed by a series of ribs, the number of which varies with 

 that of the dorsal vertebrae. In most cases each rib articu- 

 lates by its head with the bodies of two vertebrae, and by its 

 tubercle with the transverse process of one of these vertebrae 

 (the lower one). In the Monotremata {e.g., the Duck-mole), 

 the ribs articulate with the body of the vertebra only ; and 

 in the Whales, the hindermost of the ribs, or all of them, 

 articulate with the transverse processes only, and not with 

 the centra at all. 



There are usually no bony pieces uniting the ribs with 

 the sternum or breast-bone in front, as in Birds ; but the 

 so - called " sternal ribs " of Aves are represented by the 

 " costal cartilages " of the Mammals. In some cases, how- 

 ever, the cartilages of the ribs do become ossified and con- 

 stitute sternal ribs. Sometimes, as in the Armadillos, there 

 is a joint between the vertebral rib and costal cartilage. 

 More rarely, as in the Monotremes, an intermediate piece is 

 found between the vertebral and costal portions of the rib. 

 Only the anterior ribs reach the sternum, and these are 



