42 GENERAL ANATOMICAL CHARACTERS 



moreover, very often sends down near its extremity a more or 

 less compressed plate (inferior lamella), which, being considered 

 serially homologous with the ribs of the thoracic vertebra? (though 

 not developed autogenously), is often called the " costal " or 

 " pleurapophysial " plate. This is usually largest on the sixth, and 

 altogether wanting on the seventh vertebra. The first and second 

 cervical vertebrae, called respectively "atlas" and "axis," are 

 specially modified for the function of supporting and permitting 

 the free movements of the head. They are not united together 

 by the intervertebral substance, but connected only by ordinary 

 ligaments and synovial joints. 



The cervical region in mammals presents the remarkable 

 peculiarity that, whatever the length or flexibility of the neck, the 

 number of vertebrae is the same, viz. seven, with the exception of 

 the Manatee and Hoffman's Two-toed Sloth (Cholospus hoffmanni), 

 which both have but six, and the Three-toed Sloth (Bradyjms 

 tridactylus), which has nine, though in this case the last two usually 

 support movable ribs, which are not sufficiently developed to reach 

 the sternum. 



According to Parker there may occasionally be eight cervicals 

 in the Pangolins (Manis). 



Dorsal Vertebral. — The dorsal (or, as it would be more correctly 

 termed, thoracic) region consists of the vertebrae succeeding those 

 of the neck, which have ribs movably articulated to them. These 

 ribs arch round the thorax — the anterior one, and usually the 

 greater number of those that follow, being attached below to the 

 sternum. 



Lumbar Vertebrce. — The lumbar region consists of those vertebrae 

 of the trunk in front of the sacrum which bear no movable 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 undeter- 

 mined 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 lumbar vertebrae, the whole number 

 being tolerably constant in a given group of animals, and any 

 increase of the one being at the expense of the other. Thus in all 

 known Artiodactyle Ungulata there are 1 9 dorso-lumbar vertebrae ; 

 but these may consist of 1 2 dorsal and 7 lumbar vertebrae, or 1 3 

 dorsal and 6 lumbar, or 14 dorsal and 5 lumbar. The smallest 

 number of dorso-lumbar vertebrae in mammals occurs in some 

 Armadillos, which have but 14. The number found in Man, 

 the higher Apes, and most Bats, viz. 17, is exceptionally low; 

 19 prevails in the Artiodactyla, nearly all Marsupials, and very 

 many Rodents; 20 or 21 in Carnivora and most Insectivora ; 



