CHARACTERS OF MAMMALIA. 473 



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 vertebrae are anchylosed together 

 to a greater or less extent, and the neck is immovable, the 

 odontoid process is also wanting. 



In almost all Mammals the spinous processes of the dorsal 

 vertebrae are very largely developed for the attachment of the 

 structure which is known as the ligamentum nuchtz. This is a 

 great band of elastic fibrous tissue, which is attached in front 

 to the occipital bone and spinous processes of the cervical 

 vertebrae, and which relieves the muscles of the task of support- 

 ing the head, in those Mammals which progress with the body 

 in a horizontal position. The development of the ligamentum 

 nucha is consequently, as a rule, proportionate to the size 

 of the head and the length of the neck. In Whales no such 

 apparatus is necessary, owing to the fixation of the cervical 

 vertebrae by anchylosis ; and in Man, who walks erect, the 

 ligamentum nucha can hardly be said to exist as a distinct 

 structure, being merely represented by a band of fascia. 



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

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

 brae 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 vertebrae. In most cases each rib articulates 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 trans- 

 verse 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 carti- 

 lages " of the Mammals. In some cases, however, the carti- 

 lages of the ribs do become ossified and constitute sternal ribs. 

 Sometimes, as in the Armadillos, there is a joint between the 



