J. Playfair McMurrich 65 



one from the external and the otlier from tlie internal condyle, and it 

 unites with the soleus to form the tendo Achillis, inserting into the 

 OS calcis. Disregarding the soleus for the present, there are two possi- 

 bilities to be considered with reference to the double origin of the 

 gastrocnemius; either (1) it represents two originally distinct muscles 

 which have united below, or (2) it represents the splitting in its upper 

 part of an originally single muscle. The second of these possibilities 

 may be dismissed on the ground that in the lower mammals the two 

 heads, as a rule, remain distinct throughout their entire length. Eisler, 

 in accepting the view that the two heads are primarily distinct muscles, 

 takes the ground that one or the other of them has undergone an exten- 

 sive migration, basing this conclusion upon the crossing of the two 

 tendons which occurs shortly above their insertion, a peculiarity which 

 has been considered in detail by Parsons, 94. The crossing, considered 

 by itself, throws little light upon the question as to which muscle has 

 undergone the supposed migration and Eisler, turning for evidence to 

 the nerve supply, finds that Cunningham, 81, has observed in Phal- 

 angista maculata that the gastrocnemius medialis is supplied from the 

 external saphenous (sural) nerve, which has a markedly fibular position 

 and he concludes therefore that it is the gastrocnemius medialis which 

 has migrated and that primarily it had its origin from the fibula and 

 lay to the fibular side of the gastrocnemius lateralis, in which case 

 there would be no crossing of the tendons. 



The argument by which such a remarkable migration is deduced is 

 open to criticism along several lines. In the first place the crossing 

 of the tendons does not necessarily imply a migration of the muscles. 

 It may be difficult to give a satisfactory explanation of it on another 

 basis, and the migration theory, if correct, would certainly explain it, 

 but it may be pointed out that the same crossing occurs also in the 

 tendons of the flexor filjularis and the flexor tibialis digitorum in man, 

 and yet a reversal of the relative position of the two muscles by 

 migration seems altogether improbable. A theory which explains the 

 one crossing will probably also explain the other, for, it may be noted, 

 the tendons of the gastrocnemius and plantaris represent a superficial 

 layer of the plantar aponeurosis into which both muscles primarily 

 insert, while those of the two long digital flexors represent a deep layer 

 of the same aponeurosis. The most probable factor in the production 

 of the crossing is a physiological rather than a morphological one, a point 

 which will be considered later on in connection with the discussion of 

 the flexor tibialis digitorum. 



In the second place it would seem that Eisler has placed too much 



