634 MAMMALIA. 



tebrse in the neck of the Sloth is evidently to allow of a more ex- 

 tensive rotation of the head ; for, as each of the bones turns to a 

 small extent upon the succeeding one, it is clear that the degree of 

 rotation of the extreme point will be in proportion to the number 

 of pieces in the whole series. When the habits of this extra- 

 ordinary animal are considered, hanging as it does from the under 

 surface of boughs, with the back downwards, it is obvious that the 

 only means by which it could look towards the ground must be by 

 rotation of the neck ; and as it was necessary, to effect this without 

 diminishing the firmness of the cervical portion of the vertebral 

 column, to add certain moveable points to the number possessed 

 by the rest of the class, the additional motion was acquired by mo- 

 difying the two superior dorsal vertebrae, and giving them the 

 office of cervical, rather than by infringing on a rule, which is thus 

 preserved entire, without a single known exception." 



(712.) The occipital bone articulates with the atlas by two 

 lateral condyles, instead of by a single central articulating surface ; 

 a circumstance which depends upon the greatly increased develope- 

 ment of the encephalon, and the consequent expansion of the 

 cranium. 



(713.) The number of dorsal vertebrae depends upon that of the 

 ribs : thus, in the Bat tribe there are only eleven ; while in some 

 of the Pachydermata, as, for example, in the Elephant and Tapir, 

 as many as twenty dorsal vertebrae may be counted. The lumbar 

 and sacral vertebrae will likewise be more or less numerous in dif- 

 ferent genera ; and in the number of pieces composing the coccyx, 

 or tail, there is every variety, from four to five and forty. 



(714.) The thorax is enclosed by ribs, that in structure, and 

 in their mode of connection with the dorsal vertebrae, resemble 

 those of Man. At its dorsal extremity each rib is articulated by 

 its head to the bodies of the vertebrae, and to the intervertebral 

 substance ; while its tubercle, or the representative of the second 

 head of the rib of a Bird, is moveably connected with the corre- 

 sponding vertebral transverse process. There are no sternal ribs ; 

 but these are represented by cartilaginous pieces, whereby towards 

 the anterior part of the thorax each rib is attached to the side of 

 the sternum ; posteriorly, however, this connection does not exist. 

 The anterior ribs are therefore called true ribs, and the posterior, 

 false or floating ribs, precisely as in the human skeleton. 



(715.) The sternum is composed of several narrow pieces, 

 placed in a line behind each other along the middle of the breast. 



