xx.] ANTERIOR AND POSTERIOR EXTREMITY. 333 



wards, the preaxial side is outwards, and the postaxial side 

 towards the middle line of the body, and as long as the 

 radius and ulna retain their primitive parallel position, the 

 manus is placed with the ends of the digits directed back- 

 wards, the preaxial side being external. 



The hind limb (see Fig. f) is, at the same time, rotated 

 from the hip to the same extent forwards, so that the femur 

 is also nearly parallel to the axis of the body, but with the 

 knee projecting forwards ; the preaxial side is inwards and 

 the postaxial side outwards ; the tibia and fibula are parallel, 

 the former internal and the latter external ; the foot has the 

 ends of the digits directed forwards, the hallux or preaxial 

 digit is on the inner, and the fifth or most postaxial digit 

 is on the outer, side. 



In this position the hind limb remains, subject only to 

 minor modifications, in nearly all terrestrial Mammals ; but 

 the Walrus, and to a certain extent the Sea- Lions, alone 

 carry the fore limb as described above without further 

 modification. 



The next stage, affecting the fore limb alone, consists in 

 the rotation of the lower end of the radius around the ulna, 

 which brings the distal extremity of the manus round from 

 the back to the front of the limb (see Fig. g). In most 

 Mammals the limb is permanently fixed in this position, 

 and the bones of the fore-arm become greatly modified in 

 consequence, as described in Chapter XV. It will now be 

 understood how, though the outer side of the humerus 

 corresponds with the inner side of the femur, in ordinary 

 quadruped progression, yet the outer side of the manus 

 corresponds with the outer and not the inner side of 

 the pes. 



To these general conditions there are certain modifications 

 met with in some animals, and certain exceptions in others. 



