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. ^''^' 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 adherence 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 

 ,_„„_„„. T can be identified for each region. The vertebrae 

 REGIONS. ^""^ ^'^'^ arranged m a straight hne; 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 in a lighter 

 medium. The curvatures are modified in man, in view of the 6rect 

 position, in that there is a lumbar curvature ventrad counteracting 

 the effect of the dorsal curvature of the thorax, while the caudal 



