38 



The Phylogeny of the Crural Flexors 



to its destination, but that route need not be exactly the same in all forms. 

 The tendency, however, is largely toward;^- conservatism, and even when 

 the bulk of the fibers composing a given nerve trunk, adopt, in the higher 

 vertebrates, a new path, some will be apt to retain the original course 

 and so serve as guides for the determiiiation of topographic relationships. 

 I have elsewhere (1903, pp. 466-7) expressed in general terms the con- 

 clusions in regard to the value of nerve supply in 

 determining muscle homologies to which my studies 

 of the muscle and nerves of the forearm have led me. 

 In Amblystoma, immediately above the knee joint, 

 two distinct nerve trunks occur upon the posterior 

 surface of the leg (Fig. 2). They are formed by the 

 division of the sciatic nerve after it has given off the 

 peroneal nerve and are what Humphry, 72, has 

 termed the internal and external popliteal nerves. 

 They do not, however, correspond in composition to 

 the nerves so named in the mammalia, and for this 

 reason they will be spoken of here as the ramus plan- 

 taris profundus (rp) and the ramus plantaris super- 

 ficialis (rs). The}^ lie, at first, one on either side of 

 the sciatic artery, but as they are traced downwards 

 the ramus profundus passes slightly laterally so as to 

 come to lie in front of the ramus superficialis, and a 

 little later the two stems fuse, only to separate again, 

 some interchange of fibers apparently taking place, 

 however, during the fusion, and a further interchange 

 is carried out by means of a cross connection between 

 the two stems a little lower down. 

 From the ramus superficialis above the fusion branches are given off 

 to the plantaris superficialis medialis (Psm) and to the plantaris super- 

 ficialis lateralis (Psl), and below the fusion to the plantaris profundus 

 III (Pp^^^) and the plantaris profundus II (Pp^^). Just as these 

 nerves are given off the stem is passing over the upper border of the 

 plantaris profundus III and its course is then downwards and outwards 

 between the fibulo-tarsalis and the plantaris profundus II (Fig. 1, /). 

 or to a certain extent through the substance of the iibulo-tarsalis. It 

 thus reaches the fibular side of the crus and descends towards the foot. 

 lying between the lateral border of the fibulo-tarsalis and the origin of 

 the plantaris profundus II. Beyond this it will be unnecessary to follow 

 it at present. 



The ramus profundus (rp) gives off above the fusion a branch to 



ram 



of the crural nerves 

 in jLmblystoma tigri- 

 num. C, cutaneous 

 branch ; EP, external 

 plantar; Ex, branches 

 to extensor surface ; 

 7. branch to inter- 

 osseus ; IP, inter- 

 nal plantar; P pi- III, 

 branches to plantares 

 profundi I-III ; Psui, 

 branch to plantaris 

 superficialis medial- 

 is ; rp, ramus pro- 

 fundus : IS, ramus 

 superficialis. 



