THE VERTEBRAL COLUMN AND LIMBS. 279 



This process corresponds morphologically to the centrum of the atlas, 

 which is separated from the latter and joined to the centrum of the 

 axis. 



The dorsal vertebras are characterised by high, crest-like, spinous 

 processes, and by the possession of ribs. The anterior ribs are 

 attached by cartilage to the sternum, which is usually elongated and 

 composed of a number of bony pieces arranged one behind another ; 

 the posterior ribs (the so-called "false ribs") do not reach the 

 sternum. The ribs articulate with the vertebrae by means of a 

 capitulum and a tuberculum. While the number of cervical vertebrae 

 is almost constantly seven, that of the dorsal vertebrae is subject to a 

 greater variation. As a rule there are thirteen, sometimes twelve 

 dorsal vertebrae ; but there is a less number in some Bats and Arma- 

 dillos, while there are fifteen or more in some animals. The Horse 

 has eighteen, the Rhinoceros and Elephant nineteen to twenty, and 

 the three-toed Sloths have twenty- three to twenty- four. The lumbar 

 vertebrae, which have long lateral processes in place of ribs, are 

 usually seven in number. The number rarely sinks to two as in 

 Ornithorhynchus and the two-toed Anteaters, and still more rarely 

 rises to eight or nine (Stenops). The sacral vertebrae, which vary in . 

 number from two (Marsupials) to four, more rarely nine (Armadillo), 

 are firmly united with one another, and by their transverse processes 

 (with the rudiments of the ribs) with the iliac bones. The caudal 

 vertebrae, which vary considerably in number and mobility, become 

 narrower towards the end of the axis of the body, and often 

 (Kangaroo and Anteaters) possess inferior spinous processes ; but all 

 the processes become less and less conspicuous towards the posterior 

 extremity. 



The anterior pair of extremities is never absent. The clavicle is 

 absent when the anterior limbs serve only for the support of the 

 anterior part of the body in locomotion, or perform simple, pendulum- 

 like movements, as in swimming, walking, running, jumping, etc., 

 (Whales, Ungulates, Carnivora). Otherwise the scapula is connected 

 with the sternum by a more or less strong, rod-shaped clavicle. The 

 coracoid is almost always reduced to the coracoid process of the 

 scapula ; in the Monotremata only is it a large bone which reaches / 

 the sternum. The posterior extremities are more firmly connected 

 with the body than are the anterior. In the Whales alone is the 

 pelvic girdle rudimentary, and is represented by two rib-like bones 

 which are quite loosely connected with the vertebral column. In all 

 other Mammals the pelvic girdle is fused with the lateral parts of 



