70 The Phylogeny of the Crural Flexors 



of the two muscles being but various differentiations, due to differences 

 of strain, of an originally single aponeurosis and not a secondary fusion 

 of distinct structures. 



The significance of the tibialis posticus has already been indicated; 

 from its relations and nerve supply it seems unquestionably a derivative 

 of the plantaris profundus I, a view not at variance with that of Eisler, 

 when allowance is made for the differences in our terminologies. Another 

 derivative of the same layer is the flexor accessorius digitorum pedis 

 (quadratus plantce), which represents a portion of the layer which takes 

 its origin from the tarsal bones and is inserted into the plantar aponeu- 

 rosis. The muscle certainly finds no place in the general plan of the 

 plantar muscles and is clearly represented in the lacertilia, where it is 

 supplied by a branch from the ramus profundus. Its supply from the 

 external plantar nerve in the mammalia is readily explained on the basis 

 of the separation of the plantar fibers from the ramus profundus to form 

 a special more superficial nerve stem and to subsequently unite with the 

 external plantar fibers to form the posterior tibial nerve, as has already 

 been described. Its relations to the tendon of the long digital flexor is 

 clearly a persistence of its original insertion into the deep layer of the 

 plantar aponeurosis. 



Finally as regards the popliteus, the most usually propounded homology 

 is with the uppermost portion of the interosseus muscle, and, in truth, 

 at first sight this seems to be a most plausible suggestion. There are, 

 however, some difficulties in its way, one of the most important being 

 its origin from the external condyle of the femur and another that in 

 some forms it covers in, i. e., lies posterior to the upper portions of the 

 flexor tibialis and the tibialis posticus. On the other hand, its constant 

 supply from the ramus profundus seems to imply in almost unmistakable 

 terms its derivation from either the interosseus or the plantaris pro- 

 fundus I, and of the two the interosseus seems to be its most likely 

 origin. 



Eisler, though, recognizing a possibility of referring it to the inter- 

 osseus, finally concludes that it is not properly a crural muscle at all 

 in the sense in which the term crural is used here, but that it is a femoral 

 muscle and the equivalent of the brachialis anticus of the arm. It is 

 difficult to see how such an homology can be worked out in its details. 

 It would imply that the muscle is a derivative of one of the femoral 

 flexors, most presumably of the biceps or better of such a muscle as the 

 eruro-coccygeus of .the opossum, which sends a slip obliquely across the 

 thigh to be inserted into the shaft of the tibia. It is to be noted, how- 

 ever, that this slip passes superficially to the upper part of the gastroc- 

 nemius, while the popliteus passes beneath, i. e., anterior to that muscle. 



