ADAMS, PHYLOGEXY OF THE JAW MUSCLES 149 



terior is the posterior slip from the capiti-mandibularis, which is sepa- 

 rate in birds. It crosses the anterior pterygoid and is the homolog of the 

 muscle of the same name in Eeptilia. 



The depressor mandibulaa is large and is the homolog of the depressor 

 of the Eeptilia. 



The presence of the stapedial muscle in the Aves shows that in some 

 of the reptiles and Aves the stapes was connected with a part of the de- 

 pressor mandibular Futamura (1907) has shown that this was true in 

 the embryology of the sparrow and of the duck : 



Der Stapedius entspringt von der Gehorkapsel und geht zum Columella und 

 zu der medialen Seite der Gehorgrube. Ein Teil der Muskelfasern scheint 

 mit dem proximalen Abschnitt des Quadratum Beziehung zu haben. . . . 

 In der Sperling der M. stapedius, der aus dem vorher erwalmten proximalen 

 Teil der Facialismuskulatur entsteht, ist sogar schon ziemlich deutlich differ- 

 enziert. Er liegt an der dorsalen Seite der iibrigen Facialismuskulatur und 

 zieht grosstenteils an der medialen Seite des Facialisstammes nach hinten, 

 medial, und endet dicht an der Carotiswand der Nervus facialis ist bedeutend 

 verlangert. In der Ente M. stapedius, digastricus und stylohyoideus differen- 

 zieren sich aus dem Blasten der post-auricularen Gegend. 



Homologies of the Jaw Muscles ix Mammals 

 (Table IT) 



The great transformation of the reptilian skull into the mammalian 

 skull has been correlated with equal changes in the jaw muscles. The 

 temporal fossae of mammals haye opened out, so that the temporal and 

 masseter muscles extend to the surface of the head and thus gain room 

 and free action for diverse adaptation and for expansion. By the devel- 

 opment of the sagittal and occipital crests the areas for the origin of the 

 temporal and neck muscles are greatly increased. The masseter becomes 

 enlarged and its origin spreads over the zygomatic arch. This specializa- 

 tion of the superficial part of the temporal mass is typically mammalian, 

 as nothing like it is seen in the reptiles. 



The great changes in the mandible, the reduction of the posterior jaw 

 elements, and the development from them of the malleus and incus have 

 been correlated with an equal transformation of the muscles. The inser- 

 tion of the whole temporal mass has shifted from the coronoid region of 

 the mandible to the dentary, while the external muscle gave rise to both 

 the internal and external muscles of the mammals. The formation of 

 accessory auditory ossicles from the reduced posterior jaw elements gave 

 opportunity to certain of the reptilian muscles to assume new functions. 



The stapedial muscle, for example, would be homologized with the 



