J. Playfair McMurrich 69 



origin, the two muscles standing to one another in much the same 

 relation as the flexor profundus digitorum and the flexor longus hallu- 

 cis of man. Their nerve supply is altogether different, however, since 

 the flexor fibularis is supplied by the equivalent of the ramus superfi- 

 cialis medialis, while the flexor tibialis is supplied from the ramus 

 profundus. There seems to be no good reason why this should be so if 

 the two muscles belong to the same original layer, and one is forced to 

 the conclusion that they have their origin from quite different layers. 

 In the amphibia and lacertilia it has been shown that the plantares 

 profundi III and II are supplied from the ramus superficialis medialis, 

 while the plantaris profundus I and the interosseus are supplied from 

 the ramus profundus. The flexor fibularis, accordingly, probably repre- 

 sents the plantaris profundus II, if the soleus be regarded as equiva- 

 lent to the plantaris profundus III, w^hile the flexor tibialis probably 

 represents in part the plantaris profundus I. In other words the flexor 

 tibialis is a muscle derived from the same primary layer as the tibialis 

 posticus and is quite distinct from the flexor fibularis. 



This view may seem improbable on account of the close relation of the 

 tw^o muscles in their lower portions and on account of the distinctness 

 of the tibialis posticus, but it must be remembered that the primary 

 insertion of a considerable portion of the plantaris profundus I is 

 probably into the plantar aponeurosis and that in the lacertilia it is in 

 part united to the sesamoid cartilage developed in the tendon of the 

 plantaris superficialis III-II. It is this aponeurotic portion of the pro- 

 fundus I which becomes the flexor tibialis, while the remainder of it 

 constitutes the tibialis posticus, and, as will be shown later, the flexor 

 accessorius digitorum pedis. 



Eisler finds the homologue of the flexor tibialis in his plantaris 

 profundus II and that of the flexor fibularis in his plantaris profundus I, 

 thus coinciding with the opinion expressed above that the muscles 

 belong to different primary layers and also with the identification of the 

 two muscles, since the muscle he names the plantaris profundus I 

 is identical with that which I have called the plantaris profundus II and 

 that which he calls the plantaris profundus II is a part of my plantaris 

 profundus I. 



The remarkable transference of the action of the fiexor fibularis from 

 the fibular digits to the great toe wdiich occurs in the mammalian series 

 has received its most plausible explanation from Keith, 94, on functional 

 grounds. It remains to especially emphasize in connection with his 

 argument the primary insertion of both muscles into the deeper layers 

 of the plantar aponeurosis, the different arrangements of the tendons 



