J. Playfair McMurrich 55 



sistently apijlied in the lower forms. He proposed, accordingly, and his 

 proposition was accepted by Dobson, to speak of the two muscles usually 

 recognized as the flexor longus hallucis and the flexor longus digitorum as 

 the flexor tligitorum fibularis and the flexor digitorum tibialis, respect- 

 ively. The proposition is certainly worthy of general acceptance and 

 is almost necessary from the comparative standpoint, since in the majority 

 of mammals the flexor digitorum fibularis (fl. longus hallucis) is the 

 principal muscle and the flexor tibialis the subordinate one. 



Dobson has pointed out that the relations of the two flexors is accord- 

 ing to one of two types and that all the members of any family, if not 

 order, of mammalia will present the same type. In one type the tendons 

 of the two muscles fuse, while in the other they remain distinct, and not- 

 withstanding that he found the aplacental mammalia presenting the 

 second type of relation, Dobson concludes that the first is the more 

 primitive, since, as he states it, " it is difiicult to conceive that in any 

 animals in which a definite separation of the tibial from the fibular 

 flexors had once taken place— symmetrical reunion of these tendons 

 could subsequently occur." With such a view the phylogenetic plan 

 here being traced agrees, for an important part of this plan is the recog- 

 nition of the plantar aponeurosis of the lower forms in the tendons of 

 the long flexors, all the post-axial muscles of the crus, except the inter- 

 csseus, having their insertion primarily into that aponeurosis, through 

 which their action is extended to the digits. 



The descriptions of the long flexors which Dobson has given for so 

 many species of mammals are sufficiently thorough to warrant the omis- 

 sion of a detailed description of the arrangement observed in the forms I 

 have studied, but for the sake of completeness and to bring out especially 

 their relations to the plantar aponeurosis, or rather its mammalian repre- 

 sentatives, a description of the arrangement observed in the opossum may 

 be given. 



The -jiexor fibularis digitorum (Figs. 7 and 8, FF) arises from the 

 inner and posterior surfaces of the greater portion of the fibula. In its 

 upper part it is separated by a strong aponeurosis from the adjacent 

 tibialis posticus, and at about the middle of the leg a strong aponeurosis 

 appears upon its posterior surface, separating the muscle from the more 

 superficial plantaris. Traced downwards this aponeurosis gives rise upon 

 its posterior surface to a muscle which increases rapidly in breadth, 

 while the aponeurosis diminishes in that dimension, although thicken- 

 ing to form a structure to which the term tendon is applicable. The 

 muscle is the flexor brevis digitorum, or rather a considerable portion of 

 it, and need not concern us any further except in so far as its origin from 

 5 



