MAMMALIA 



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exclusively that of the rabbit. Almost any mammalian skeleton 

 will answer the purpose, and the bones of the larger mammals 

 are more convenient to handle. 



The vertebral column arises in the scleratogenous tissue 

 around the notochord and nerve cord. The connective tissue 

 is chondrified to form centra and arches which constrict the 

 notochord and encircle the spinal cord. The cartilaginous 

 vertebrae are converted into bony vertebrae, centres of ossifica- 

 ton appearing in the centra and on each side of the arches. 

 Their origin is accompanied by the development of cartilaginous 

 ribs in the septa between the myotomes of the thoracic region, 

 and these are ultimately ossified. The ribs extend around the 

 body, and the anterior ones encircle it, and it is believed that 

 their fusion ventrally forms the sternum. At all events, the 

 sternum is preformed in cartilage and preserves to some extent, 

 and in some mammals to a great extent, its cartilaginous 

 structure through life. 



In the adult the vertebral column consists of a large number 

 of vertebrae articulated together to form an elastic support to 

 the trunk and tail. According to the position they occupy, 

 they are resolved into cervical, thoracic (dorsal or costal), 

 lumbar, sacral, caudal. The formula for the rabbit and hare 

 is C. 7, Th. 12 (or 13), L. 7 (or 6), S. 3, Cd. about 15. Compar- 

 ing the skeletons of other mammals it will be found that the 

 cervical almost always number seven, while the other kinds 

 vary in number. Here are a few examples : 



A lumbar vertebra consists of a body, or centrum, on the 

 dorsal aspect of which a neural arch encircles the neural canal. 

 It is produced medianly into a neural spine (neuropophysis), 

 and laterally into a pair of large transverse processes. The 



