398 MAMMALIA. 



and the Dugongs and Manatees (Sirenia}, the vertebral column 

 is divisible into the same regions as in man namely, into a 

 cervical, dorsal, lumbar, sacral, and caudal or coccygeal region 

 (see fig. 269). In the Cetacea and Sirenia the dorsal region of 

 the spine is followed by a number of vertebrae which compose 

 the hinder extremity of the body, but which cannot be separ- 

 ated into lumbar, sacral, and caudal vertebras. 



In spite of the great difference which is observable in the 

 length of the neck in different Mammals, the number of verte- 

 brae 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 ex- 

 ceptions to this law are the Manatus anstralis, one of the Sea- 

 cows, which has usually six cervical vertebras ; and the three- 

 toed Sloth (Bradypus tridactylus), which is commonly regarded 

 as possessing nine, though competent anatomists would refer 

 the posterior two of these to the dorsal region. 



The dorsal vertebras 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 three-fingered Sloth the 

 maximum is attained. The lumbar vertebras 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 cavi- 

 ties for the reception of the two condyles of the occipital bone, 

 and the second vertebra, or axis, usually has an " odontoid " 

 process on which the head rotates. In the true Whales, how- 

 ever, in which the cervical vertebras are anchylosed together to 

 a greater or less extent, and the neck is immovable, the odon- 

 toid process is also wanting. 



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

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

 bras 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- 

 limbs are wanting, and the pelvis rudimentary, there is no 

 " sacrum." 



The thoracic cavity or chest in Mammals is always enclosed 

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

 dorsal vertebras. In most cases each rib articulates by its head 

 with the bodies of two vertebras, and by its tubercle with the 

 transverse process of one of these vertebras (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 



