THE MUSCULAR SYSTEM 



243 



parts as given above and leads them to make the comparison 

 in other ways ; thus, in the earliest of these theories, more than 

 a century ago, the right arm was compared, not with the right- 

 leg, but with the left; the thumb became thus the homologue 



D 



FIG. 65. Diagrams explanatory of various theories of limb homology. 



(A) Syntropist theory, members of the same side homologized. (B) Antitropist 

 theory, members of opposite sides homologized. (C) Homology between the spinal 

 nerves involved in the antitropist theory. The cervical and dorsal nerves going 

 posteriorly, are compared with the lumbo-sacral nerves going anteriorly. Thus, the 

 fourth cervical nerve (C 4 ) is the homolog of the second sacral (S 2 ), and so on. (D) 

 Theory of FOLTZ, 1863. The first digit of each limb is bivalent, and the equivalent 

 of digits 4+5 of the other. (E) Theory of EISLER, 1895. Here the relation is a.n- 

 titropic, but the homology applies to the three inner digits only in each member, 

 leaving no homologue for digits 4 and 5 in each case. 



of the little toe, radius was compared with fibula, and ulna 

 with tibia (Fig. 65, B). That this theory, fantastic as it may 

 seem, is not merely a vague speculation, but one to the aid 



