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Anatomy of Skeleton 



This is. the usual division, but occasionally exceptions may be encountered : 

 thus there may be twenty-three or twenty-five presacral vertebrae, consequent on 

 forward or backward shifting of the pelvis, and this may be correlated with change in 

 the number of sacral vertebrae, or the sacral may be altered in number without an 

 alteration in the presacral number. Again, the number of dorsal vertebrae may appear 

 to be altered by the occurrence of a rib on the last cervical segment, or on the first 

 lumbar, or by partial suppression of the first or last of its own series, but these are not 

 as a rule really alterations in number, but only in appearance. The coccygeal verte- 

 brae^are inconstant in number, as they are in a state of degeneration, representing 



/. 



Ceqv/CAL. 



FIG. 9. Vertebrae taken from the three presacral regions to show the characters of these regions 

 Cervical and lumbar vertebrae seen from above, dorsal from the side 



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the bony skeleton of the tail : in the embryo there are six of these rudiments to be 

 found. 



In the various regions the true vertebrae exist under different physical conditions 

 and accordingly exhibit distinguishing modifications/ and the distinction can be even 

 carried so far as to enable the observer to recognise and place the individual vertebra 

 in their proper order in most cases. 



The different groups can be known at once by their distinguishing characters. 

 Thus the cervical vertebrae possess an arterial foramen in their " transverse processes," 

 the dorsal group carry free ribs and therefore present costal facets on their bodies and 

 (in the majority) on their transverse processes, while the lumbar vertebrae have neither 



