J. Playfair McMurrich 67 



version of the deeper and more superficial muscles, his fibulo-plantaris 

 and plantaris superficialis minor coming to lie on a plane posterior to his 

 plantaris superficialis major. Such a transposition can only be 

 accepted on the strongest evidence, and of this, it seems to me, there is a 

 failure. 



Finally, as was pointed out in considering the antibrachial fiexors, 

 any theory which requires the migration of a muscle origin over a joint 

 from below demands the closest scrutiny. Eisler's homologies make the 

 gastrocnemius medialis have its origin primarily from the head of the 

 fibula, and to reach the position it has acquired in the lacertilia and 

 mammalia it must have migrated upwards over the knee joint as well 

 as medially. If a plausible homology can be set forth which does not 

 require this migration, the presumption is in its favor. In the forearm 

 it was shown that the palmaris superficialis layer was distinguished from 

 the other flexor layers by having its origin from the humerus, and that 

 throughout the whole series of forms studied it retains that origin. The 

 remarkable similarity which obtains between the amphibian antibrach- 

 ium and crus leads to the expectation that in all probability the homo- 

 logue of the superficial palmar layer will have the same relations, and the 

 identification of the plantares superficiales medialis and lateralis with 

 the gastrocnemii exactly fulfills the expectation. 



The conclusions to which I have been led, then, are that the gastroc- 

 nemius medialis and lateralis of the mammalia are primarily separate 

 muscles which insert into the superficial layer of the plantar aponeu- 

 rosis, and that they represent the greater part of the superficial plantar 

 layer of the amphibian crus, the gastrocnemius medialis corresponding 

 to the plantaris superficialis medialis of both amphibia and lacertilia 

 and the gastrocnemius lateralis to a portion of the amphibian plantaris 

 superficialis lateralis and to the lacertilian muscle similarly named. 



The plantaris. — There can be little doubt but that the plantaris is 

 a derivative of the same muscle mass which gives rise to the gastroc- 

 nemius lateralis, or, to be more precise, that it represents the deeper 

 medial portion of that mass. For it is typically associated with the 

 gastrocnemius lateralis and is frequently united with that muscle in its 

 upper part, occupying then the position indicated. It is already a dis- 

 tinct muscle in the reptilia, at least the muscle described above as the 

 plantaris superficialis accessorius seems to be its homologue, although 

 the relations which this muscle bears to the plantaris profundus III 

 seems at first sight to preclude any such homology. But it must be 

 remembered that after all the association is not directly with the pro- 

 fundus III, but with the plantar aponeurosis into which the profundus 

 III also inserts. 



