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 vertebre (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 vertebre is the same, viz. seven, with the exception of 
the Manatee and Hoffman’s Two-toed Sloth (Cholepus hoffmannt), 
which both have but six, and the Three-toed Sloth (Bradypus 
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 Vertebree.—The dorsal (or, as it would be more correctly 
termed, thoracic) region consists of the vertebra 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 Vertebree.—The lumbar region consists of those vertebree 
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. Asa 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 19 dorso-lumbar vertebree ; 
but these may consist of 12 dorsal and 7 lumbar vertebra, or 13 
dorsal and 6 lumbar, or 14 dorsal and 5 lumbar. The smallest 
number of dorso-lumbar vertebre 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 ; 
