28 THE VERTEBRATE SKELETON. 



MODIFICATIONS IN THE POSITIONS OF THE LIMBS'. 



In their primitive position the limbs are straight and are 

 extended parallel to one another at right angles to the axis of 

 the trunk. Each limb then has a dorsal surface, a ventral 

 surface, an anterior or pre- axial edge, and a posterior or 

 postaxial edge. 



In the anterior limb the radius and the pollex are pre- 

 axial, the ulna and the fifth finger are postaxial. In the 

 posterior limb the tibia and the hallux are pre-axial, the fibula 

 and the fifth toe are postaxial. The Cetacea and various extinct 

 reptiles, such as Ichthyosaurus and Plesiosaurus, have their 

 limbs in practically this primitive position. 



The first modification from it is produced by the bending 

 ventrally of the middle segments of both limbs upon the 

 proximal segments, while the distal segment is bent in the 

 opposite direction on the middle segment. Then the ventral 

 surfaces of the antibrachium and crus come to look inwards, 

 and their dorsal surfaces to look outwards. The brachium 

 and manus, thigh and pes still have their dorsal surfaces 

 facing upwards and their ventral surfaces facing downwards 

 as before, and the relations of their pre- and postaxial borders 

 remain as they were. Many Amphibians and Reptiles, such 

 as tortoises, carry their limbs in this position. 



In all higher vertebrates, however, a further change takes 

 place, each limb is rotated as a whole from its proximal end, 

 the rotation taking place in opposite directions in the fore and 

 hind limbs respectively. The anterior limb is rotated back- 

 wards from the shoulder, so that the brachium lies nearly parallel 

 to the body, and the elbow points backwards, the antibrachium 

 downwards, and the manus backwards ; the pre-axial surface of 

 the whole limb with the radius and pollex now faces outwards, 

 and the postaxial surface with the ulna and fifth finger now faces 



1 This account is based on Chapter XX. of Flower's Osteology of the 

 Mammalia. London 1876. 



