50 ANATOMY OF THE RABBIT 



those of the shoulder and hip, a bone is able to move in various 

 directions about its base of attachment, although actually, in the 

 limbs, this motion is almost restricted to an anteroposterior di- 

 rection. In the ginglymus or hinge-joint, as exemplified by the 

 distal articulations of the limb, motion is restricted to a single 

 plane. The gliding joint or arthrodia is one in which a slight degree 

 of motion is made possible by one surface slipping over the other; 

 it is exemplified in the accessory articulations of the vertebral 

 arches. 



Skeletal Regions 



The internal skeleton of a vertebrate nearly always consists of 

 a principal or axial portion and an appendicular portion. The 

 axial skeleton is formed by the vertebral column, the ribs, the 

 sternum, and the skeleton of the head; the appendicular skeleton 

 by the parts devoted to the support of the limbs, though in the 

 case of terrestrial vertebrates it would be more precise to say that 

 these are devoted to the support of the body on the limbs and to 

 locomotion. In both fore and hind limbs, the skeletal support 

 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 



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 a characteristic type 

 of vertebra can be identified for each region. In fishes, which live 

 in a medium of about the same weight as their bodies, the line of 

 the vertebral column is straight, and there is little indication of 

 regional differentiation. In terrestrial vertebrates, on the other 

 hand, especially in mammals, the vertebrae are not arranged in a 

 straight line but form a curve dorsad in the trunk or thoraco- 



