G8 The riiylogeny of the Crural Flexors 



As a result of the difference in the views of Eisler and myself regard- 

 ing the amphibian homologues of the gastrocnemii, a difference also exists 

 as to the hoinologne of the phmtaris. Eisler finds it in his plantaris 

 superficialis major, a muscle which, so far as its greater part is concerned, 

 Is fibular in origin and has been termed above the plantaris superficialis 

 III. Acceptance of Eislcr's homology would again require the migra- 

 tion of a muscle from below over the knee joint and, furthermore, as 

 has already been pointed out, a transition of the planes occupied by the 

 plantaris and the gastrocnemius, both of which phenomena the homo- 

 logy which I have deduced avoids. 



It may be added (1) that the primary connection of the plantaris 

 below is with the plantar aponeurosis, its insertion into the os calcis in 

 man being a secondary condition, and (2) that its frequent absence is 

 probably more correctly to be regarded as a failure to separate from the 

 gastrocnemius lateralis, in connection with which idea its not unfrequent 

 union with the tendo Achillis is of significance. 



The soleiis, the third element in the triceps surge, is a muscle at first 

 sight apparently peculiar to the mammalia, and among these is possibly 

 unrepresented as a distinct muscle in the monotremes." It has been 

 described as lacking in a number of mammals, in such cases being 

 probably included in the gastrocnemius lateralis. It has character- 

 istically an origin from the fibula and this points strongly to its being 

 a representative of the plantaris profundus group of muscles. The con- 

 ditions in the lacertilia throw little light upon the question, but it is 

 to be noted that the two superficial profundus layers are fused together 

 in these forms. They are, however, clearly distinguishable in the amphi- 

 bia and it is possible that they again become separated in the mammalia, 

 a series of modifications similar to those which occur in the antibrachial 

 flexors taking place. If this supposition be correct then it seems prob- 

 able that the soleus represents the plantaris profundus III of the 

 amphibia. The forms which I have studied do not furnish sufficient 

 data for certainty as to this homology, but it seems to be the only one 

 consonant with the facts at our disposal. Possibly a renewed study of 

 the monotreme crus with this idea in mind may yield some light. Eisler, 

 it mav be added, regards the soleus as a derivative of the gastrocnemius 

 lateralis. 



The -flexor fihidaris and the flexor tibialis are so closely associated that 

 at first one would have little hesitation in assigning them to a common 



° The erroneous application of the term soleus to the ipuscle which arises 

 from the epiphysial process of the fibula has already been noted. 



