THE SKELETAL SYSTEM. 51 
THE SKELETAL SYSTEM. 
‘ 
The designation ‘“‘vertebrate’’ has reference to a common 
feature of higher animals, fishes, amphibians, reptiles, birds and 
mammals—the possession of a backbone or vertebral column, 
composed of individual segments, the vertebrae. Vertebrates 
VERTEBRATE ANIMALS. 2°: however, more properly described 
as animals having an internal skeleton. 
The vertebral column is part of the principal, axial skeleton 
(Fig. 24), otherwise formed by the ribs, sternum, and skeleton of the 
head. In addition to the axial skeleton, nearly all vertebrates 
possess an appendicular part of the skeleton devoted to the 
support of the limbs, or in terrestrial vertebrates, more correctly 
speaking, to the support of the body on the limbs, and to locomotion. 
In both fore and hind limbs, this consists of a proximal portion 
lying within the contour of the body and forming the pectoral 
and pelvic girdles, and of a distal portion, lying beyond the general 
contour of the body and comprising the skeleton of the free 
extremities. The limbs of vertebrates present an extraordinary 
range of adaptations, being modified in the various groups into 
fins, paddles, wings, and walking or running limbs. In the majority 
of cases their adh+rence to a common ground plan is evident from 
their composition. 
The vertebral column of the rabbit consists of 7 cervical, 12 
thoracic, 7 lumbar, 4 sacral, and 14-16 caudal vertebrae. The 
vertebrae are found to be gradually modified from any intermediate 
part of the column forward or backward, but characteristic vertebrae 
VERTEBRAL can be identified for each region. The vertebrae 
REGIONS. are not arranged in a straight line; on the other 
hand, there is a curvature dorsad in the thoraco- 
lumbar region, and corresponding curvatures ventrad in the cervical 
and caudal regions. In fishes the line of the vertebral column is 
straight, and there is little indication of regional differentiation. 
The feature of curvature appears in terrestrial vertebrates, espec- 
ially mammals, and is related to the support of the body ina lighter 
medium. The curvatures are modified in man, in view of the erect 
position, in that there is a lumbar curvature ventrad counteracting 
the effect of the dorsal curvature of the thorax, while the caudal 
