74 The Phylogeny of the Crural Flexors 



amphibia are readily distinguishable. The superficial layer has increased 

 greatly in size and shows a differentiation into several muscles. The 

 second and third layers have fused and the fourth layer has differen- 

 tiated into two separate muscles. The fibulo-tarsalis has disappeared 

 and the muscles have in general a more vertical direction than in the 

 amphibia. 



5. The arrangement of the nerve trunks in the lacertilian crus is essen- 

 tially the same as in the amphibia. The separation of the prseaxial and 

 postaxial fibers takes place, however, above the knee joint. 



6. In the mammalia the same layers of muscles can be distinguished 

 although they have undergone greater differentiation into individual 

 muscles than in the lower forms. 



7. The plantar fibers of the ramus profundus are separated in the mam- 

 malia from the crural fibers and in the opossum form a more superficial 

 stem, the internal plantar, which traverses the crus without taking part 

 in its nerve supply. The other rami remain practically unaltered. In the 

 higher mammalia a further change takes place in that the ramus fibu- 

 laris (external plantar) and the internal plantar unite to form a single 

 stem, the posterior tibial, and, in man, some of the fibers belonging to 

 the ramus superficialis mediales and the ramus profundus become in- 

 cluded in this. 



8. The superficial layer of muscles retains throughout its origin from 

 the femur and the deep layers theirs from the crural bones, with one 

 apparent exception. Furthermore the insertion into the plantar apo- 

 neurosis is largely retained, although some shifting to the bones occurs. 



9. The soleus represents the second layer of muscles and its absence 

 in certain forms is probably, due to its inclusion in the gastrocnemius 

 lateralis. 



10. The flexor fibularis and flexor tibialis belong to different layers, 

 the former representing the third layer, while the latter is formed from 

 a portion of the fourth layer, as is also the tibialis posticus. 



11. The flexor accessorius digitorum (quadratus plantse) is primarily 

 one of the crural muscles and represents another portion of the fourth 

 layer of muscles. 



12. The popliteus is a compound muscle, being formed of a portion 

 from the superficial layer, united with a portion of the fifth layer. 

 The occasional occurrence of a distinct m. peroneo-tibialis in the higher 

 mammalia is probably due to a failure of the two portions to unite. 



Anatomical Laboratory, University of Michigan, July 26, WO.). 



