xx.] ANTERIOR AND POSTERIOR EXTREMITY. 369 



turned downwards. The skeleton of this limb, therefore, 

 and that of the fore-limb of the Cetacean, being retained 

 normally in almost exactly similar positions, are well adapted 

 for demonstrating the correspondence between the re- 

 spective bones of which they are composed (see Figs. 

 H. and i.). 



The necessity of the modifications in the direction of the 

 axes of the heads of the humerus and femur spoken of pre- 

 viously will easily be understood by a consideration of the 

 relative positions that these bones are adapted to assume. 

 Thus the axis of the head of the humerus in the majority of 

 Mammals is inclined towards the post-axial side of the shaft 

 of the bone, while that of the femur is inclined towards 

 its pre-axial side. 



Hitherto nothing had been said about the shoulder 

 and pelvic girdles, because the correspondence of their 

 parts is not so easily explained, nor so generally recog- 

 nised, as that of the segments of the limb proper. The 

 following view appears to be, of those yet suggested, the 

 most probable. 



It has been already shown (Chapters XIV. and XVII.) 

 that the lateral half of each girdle consists primarily of a 

 bar or rod placed vertically, and divided into an upper and 

 a lower segment, the point of attachment of the limb being 

 close to the junction of these two segments. The upper 

 segment in the fore-limb is the scapula, in the hind-limb the 

 ilium ; the lower segment in the fore-limb is the coracoid, 

 in the hind-limb the ischium and pubis. 



In every Mammal both scapula and ilium may be re- 

 solved into bars or rods of three-sided or prismatic form. 

 The two extremities of each bar are placed, as regards the 

 general position of the trunk, dorsally and ventrally. The 

 dorsal or upper extremity is capped by the supra-scapular 



B B 



