xx.] ANTERIOR AND POSTERIOR EXTREMITY. 367 



knee projecting forwards ; the pre-axial side is inwards and 

 the post-axial 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 pre-axial 

 digit is on the inner, and the fifth or most post-axial 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. c.). 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 modifica- 

 tions met with in some animals, and certain exceptions 

 in others. 



The modifications with regard to the anterior extremity 

 are that the humerus may be quite horizontal, or its distal 

 end may incline upwards, or, as is much more frequently the 

 case, it may incline somewhat downwards, so that the dorsal 

 surface is posterior and the ventral surface anterior ; the fore- 

 arm in the ordinary resting position may be quite vertical, 

 or inclined with its upper end backwards ; the whole of the 



