24 THE VERTEBRAL COLUMy. [chap. 



attachments commonly coalesce, and the rib is attached 

 solely to its corresponding vertebra. 



3. The Lumbar region consists of those vertebrae of the 

 trunk in front of the sacrum (to be afterwards defined) which 

 bear no moveable ribs. It may happen that as the ribs 

 decrease in size posteriorly, the last being sometimes 

 more or less rudimentary, the step from the thoracic to the 

 lumbar region may be gradual and rather undetermined in 

 a given species. But most commonly this is not the case, 

 and the distinction is as well defined here as in any other 

 region. 



As a general rule there is a certain relation between the 

 number of the thoracic and the lumbar vertebras, the whole 

 number being tolerably constant in a given group of 

 animals, any increase of the one being at the expense of 

 the other. Thus in all known Artiodactyle Ungulata there 

 are 19 thoracico-lumbar vertebrcC ; but these may consist of 

 12 thoracic and 7 lumbar, or 13 thoracic and 6 lumbar, or 

 14 thoracic and 5 lumbar. 



The smallest number of thoracico-lumbar vertebrae in 

 Mammals occurs in some Armadillos, which have but 14. 

 Ihe number found in Man, the higher Apes, and most Bats, 

 viz, 17, is exceptionally low ; 19 prevails in the Artiodactyles, 

 nearly all Marsupials, and very many Rodents ; 20 or 21 in 

 Carnivora and most Insectivora, 23 in Perissodactyla. The 

 highest and quite exceptional numbers are in the two-toed 

 sloth (Chola'J)us), 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, 



