Syimnetry and Homology in Limbs 113 



the two limbs may be readily determined, for, normally, they will 

 have the same relative position in both limbs, and be symmetri- 

 cally opposed to each other. Thus, the humerus and femur are 

 considered homotypes, and symmetrical rather than parallel bones, 

 because their normal will then be their natural position, the axis 

 of the humerus inclining backward, and that of the femur forward ; 

 all the corresponding parts facing in opposite directions. Dr 

 Wyman combats, at length, the views of Owen and Humphrey 

 upon the homologies of the bones of the fore-arm and leg, trusting 

 to the relative position of the two as the safest means of deter- 

 mining homologies. " If," writes Dr Wyman, " the two bones of 

 the fore-arm and of the leg are placed in planes at right angles to 

 the axis of the body, those bones must be considered homotypes 

 which occupy corresponding positions. The bone on the outside 

 of the fore-arm — viz., the radius — can only be the homotype of that 

 on the outside of the leg — viz., the fibula. But few anatomists 

 have made any allowance for the pronation of the fore-arm, and 

 most of them overlook the fact, that the proper position of the 

 bones of this segment, for comparison with those of the leg, is 

 supination. If the position of pronation is to be retained for the 

 fore-arm, the leg should go through a corresponding rotation in 

 the opposite direction. Viewed in connexion with the idea of 

 symmetry, the homotypes are determined without difficulty." The 

 patella is considered a sesamoid bone. 



In the hands and feet the homologies are not so apparent ; there 

 can be no question of the correspondence of the wrist and ankle, 

 but the relation of their several parts present peculiar difficulties. 

 Leaving form out of question, and comparing those bones which 

 occupy symmetrical positions, we must consider the inner bone of 

 the wrist articulating with the ulna, the homotype of the inner 

 bone of the ankle articulating with the tibia. In this way Pro- 

 fessor Wyman homologizes the pyramidale with the scaphoid, the 

 lunare with the calcis, the scaphoid with the astragalus, the unci- 

 form with the first cuneiform, the magnum with the second cunei- 

 form, the trapezoid with the third cuneiform, and the trapezium 

 with the cuboid ; the pisiform he is inclined to place among sesa- 

 moid bones. 



The homologies of the thumb and great toe are still obscure ; 

 the difficulties surrounding them cannot, in Dr Wyman's opinion, 

 be satisfactorily met. We notice, on the one hand, throughout 

 almost the whole series of vertebrates, the constancy with which 



