THE MUSCULAR SYSTEM 237 



aspect, and furnish a -flexor and an extensor for each side, are 

 continued with some modifications in higher animals. The 

 origin from the distal end of the humerus remains the same, 

 but the insertions along the shafts of ulna and radius are given 

 up, and are either confined to the carpal bones of the corre- 

 sponding sides or a new tendinous insertion is acquired which 

 extends to the base of some metacarpal, the muscles becoming 

 flexor carpi radialis, extensor carpi ulnaris, and so on. The 

 extensor carpi radialis of mammals becomes divided into two 

 similar muscles, longus and brevis, which insert into the bases 

 of metacarpals II and III respectively. 



A final group of limb muscles are the pronator and supinator, 

 which give the limb the power of turning about its axis, thus 

 crossing or tending to cross the two bones of the forearm or 

 lower leg. Of these the pronator lies upon the flexor, the 

 supinator upon the extensor aspect of the limb, their fibers 

 extending diagonally across from one bone to the other. In 

 Necturus there is one upon each aspect, the character of which 

 suggests their derivation from the primary system of abductors 

 and adductors. 



The striking correspondence in many features between the 

 anterior and posterior limb, especially shown in cases in which 

 the two are used in a similar way, has naturally led to the 

 theory of the serial homology between them, that is, an original 

 homology, not between different animals, as is usually meant 

 by the term, but between different parts of the same animal. 

 The theory presupposes a time at which both sets of limbs were 

 exactly alike, part for part, and thus the final results, however 

 unlike in the two cases, are referable to a single ground plan 

 from which both have been derived. It would be thus possible 

 to homologize bone for bone, muscle for muscle, and to extend 

 the parallelism to the vessels and nerves as well. 



A strong proof of this is afforded by the close similarity of 

 the two limbs in the lowest of the amphibians, as stated above, 

 for here, as shown especially in that form which is probably 

 the most primitive of all, the correspondence is very remark- 



