244 HISTORY OF THE HUMAN BODY 



of which many facts may be invoked, is shown by its per- 

 sistence in one form or another even to the present day. 



In fact, the theorists on the subject of limb homology have 

 been well divided into two schools, syntropists and antitropists, 

 the former making a direct comparison of the limbs of the same 

 side, with the digits in their usual order, the latter changing 

 the order either by reversal, by comparing the limbs upon 

 opposite sides of the body, or by some other unusual means. 

 Of this there is every possible variation ; one theory considers 

 in the first place the limbs of the same side to be the symmetri- 

 cal equivalent of each other, and that thus the ulna is the 

 homologue of the tibia and the radius that of the fibula, and 

 considers also the three radial fingers to be the equivalent of 

 the three tibial toes, but in the reverse direction, as indicated 

 in the diagram at E. This leaves the two outer digits of each 

 member without correspondence in the other. Another theory 

 compares the digits, also in the reverse order, but considers both 

 the thumb and the great toe bivalent, that is, equal to two 

 digits, and thus compares each with two other digits of the 

 other member D. This comparison of the digits in the reversed 

 direction, however, when carried to its conclusion, leads also 

 to the homologizing also in the reversed direction, of the spinal 

 nerves that supply the limbs. Thus the nerves of the brachial 

 plexus proceeding posteriorly, must be the homologues of the 

 nerves of the lumbo-sacral plexus, proceeding anteriorly, as in 

 C. Perhaps the most recent of the theories in which there 

 is a reversal of any part is one in which the limbs of the same 

 side are taken for the comparison, and in the normal position 

 as far as the knees, but which assumes that in the distal portion 

 there has been a torsion of both arm and leg, thus causing the 

 original extensor muscles to become flexors and vice versa. 

 This homologizes the flexors of the upper arm with the exten- 

 sors of the thigh, but allows in the distal portion a direct com- 

 parison of the flexors with flexors and extensors with ex- 

 tensors. 



As already suggested, the theories just enumerated are not 

 mere vague surmises, but rest in most cases upon careful study 



