J. Playfair McMurrich 63 



surface, but that in the mammalia its palmar fibers became associated 

 with the median nerve, its antibrachial portion persisting as the anterior 

 interosseous nerve. Apparently a somewhat similar process has taken 

 place in the crus. The tibial plantar fibers have separated themselves 

 from the ramus profundus and have taken a more superficial course to 

 form, in the opossum, the internal plantar nerve, though it can hardly 

 be said that they have united with ramus superficialis medialis, which is 

 represented by the branches to the plantaris soleus and flexor fibularis, 

 together with the branches given off higher up to tlie two gastrocnemii. 



The condition in the opossum does not, however, complete the re- 

 arrangement which is characteristic of the mammalia as a group, a 

 further modification consisting in the union of the internal plantar fibers 

 of the marsupial with the ramus superficialis fibularis (external plantar) 

 to form the posterior tibial nerve. It is noteworthy, however, that even 

 although this fused stem appears to be the prolongation of the internal 

 popliteal, yet, in the mouse and cat, the ramus profundus arises from 

 it at the knee joint and that in these forms it is proper to describe the 

 internal popliteal as dividing into the ramus profundus and the posterior 

 tibial, notwithstanding the discrepancy in the sizes of the two nerves. 

 Furthermore the branches for the superficial muscles arise high up, 

 some of them from the internal popliteal before it branches, while others 

 may arise either at the point of bifurcation or even from the upper part 

 of the ramus profundus. 



Finally, it may be added, that in man a further modification occurs 

 in the inclusion in the posterior tibial of certain of the fibers of both the 

 ramus superficialis medialis and the ramus profundus, namely, of the 

 former branches to the soleus and to the flexor fibularis, and of the latter 

 a branch to the tibialis posticus and that to the flexor tibialis. Indica- 

 tions of the original conditions are, however, still to be seen in the origin 

 in the popliteal space of a nerve which sends a branch to the popliteus 

 and another to the tibialis posticus and is then continued down the crus, 

 partly in the substance of the interosseus membrane, to end in the neigh- 

 borhood of the ankle joint. This nerve, whose terminal prolongation 

 down the crus was first thoroughly described by Halbertsma, 47, as the 

 n. interosseus cruris, is very evidently equivalent in its topographical 

 relations to the ramus profundus of the lower forms, although some of its 

 fibers destined for the tibialis posticus have separated from it and have 

 joined the posterior tibial nerve. It therefore represents one of the 

 primary branches of the internal popliteal and is deserving of more special 

 mention than is accorded to it in the text-books of human anatomy. 



There occurs then in the vertebrate series a progressive modiflcation 



