ADAMS, PHYLOOENY OF THE JAW MUSCLES 59 



mentions two cases worked out by Sir William Turner where the long 

 buccal nerve proceeded from the superior instead of the inferior maxillary 

 of the fifth nerve. He also gives some observations of his own on Ehphas, 

 Hyrax, and Castor, where the internal plantar nerve invades the terri- 

 tory of the external plantar and seizes upon muscles which usually do 

 not belong to it. In the fox-bat the opposite occurs and the external 

 plantar lays hold upon a muscle which under typical conditions is con- 

 trolled by the internal plantar. This piracy of the nerve terminals finds 

 its analogue in the capture of branches of the carotid, as described by 

 Tandler. This may well be considered a changing of the paths of the 

 fibers and not a change of the ganglionated cells and of the muscle fibers. 

 In the case of certain muscles of doubtful homology in Ornithorhynchus 

 Euge has shown a substitution of the nerve supply from a different 

 plexus from that which supplies the supposedly homologous muscles in 

 other mammals. He solves the problem by deciding that the muscles 

 concerned are not homologous. 



Gadow gives some cases of truly homologous muscles being supplied 

 in different types by a different plexus. He shows that, in Iguana, the 

 ischio-femoral. muscle is supplied by the ischiadic plexus; that in the 

 Crocodile it is supplied by another nerve, the obturator; while in Varanus 

 it is supplied by both. In placental mammals the adductor magnus is 

 innervated by two nerves, one from the obturator nerve and one from 

 the sacral plexus. In marsupials the adductor magnus is supplied solely 

 by the sacral plexus. 



Cunningham offers the following possible explanations of these anoma- 

 lies: 



(1) Complete obliteration, and then complete reconstruction of both nerves 

 and muscles, the muscle assuming its old origin and insertion. 



(2) Retention of both nerve and muscle elements but the adoption of new 

 and more convenient paths. 



(3) A retention of the muscular elements but a substitution of new nerve 

 elements. 



He rejects the first, does not give much consideration to the third and 

 seems to agree with Fiirbringer "that the nerve supply is the most im- 

 portant and indispensable guide but is not infallible." 



Goodrich (1909, p. 82) concludes that, "in a series of metameric myo- 

 tomes and nerves each motor nerve remains, on the whole, faithful to 

 its myotome throughout the vicissitudes of phylogenetic and ontogenetic 

 modifications." 



In the case of the jaw muscles experience shows the great importance 

 of the nerve supply in determining the homologies of muscles. 



