30 THE VERTEBRAL COLUMN. [CHAP. 



Carnivora and most Insectivora, 23 in Perissodactyla. The 

 highest and quite exceptional numbers are in the two-toed 

 sloth (Cholozpus) 27, and Hyrax 30. 



The prevailing number of rib-bearing vertebras is 12 

 or 13, any variation being generally in excess of these 

 numbers. 



4. The Sacral region offers more difficulties of definition, 

 especially at its posterior portion. 



Taking the human " os sacrum " as a guide for comparison, 

 it is generally denned as consisting of those vertebrae, between 

 the lumbar and caudal regions, which are ankylosed together 

 in the adult state to form a single bone. It happens, how- 

 ever, that the number of such vertebrae varies in different 

 individuals of the same or nearly allied species, especially as 

 age advances, when a certain number of the tail vertebrae 

 generally become incorporated with the true sacrum. 



A more certain criterion is derived from the fact that some 

 of the anterior vertebras of the sacral region have distinct 

 additional (pleurapophysial) centres of ossification, between 

 the body and the ilium (see Fig. 6, p. 26). To these perhaps 

 the term sacral ought properly to be restricted, the remaining 

 ankylosed vertebrae being called pseudo-sacral, as suggested 

 by Gegenbaur. Our knowledge of the development of the 

 sacrum in different animals is not sufficient at present to 

 apply this test universally, but it appears probable that two 

 is the most usual number of true sacral vertebrae, as thus 

 denned in the Mammalia. 



5. The Caudal Vertebra are those placed behind the 

 sacrum, and terminating the vertebral column. They vary 

 in number greatly, being reduced to 5, 4, or even 3, in a 

 most rudimentary condition, in Man, some Apes and Bats, 

 and being numerous and powerfully developed with strong 



