ADAMS, PHYLOGENY OF TEE JAW MUSCLES 153 



General Summary of Homologies and Evolution of the Jaw 

 Muscles in the Vertebrates 



The following conclusions may be regarded as well established: 



(1) The two chief muscle masses of the jaw, (a) the adductor mass 

 innervated by the ramus mandibularis (V 3 ) and (6) the depressor or 

 digastric mass innervated by the facialis (VII), are homologous through- 

 out the Vertebrata. 



(2) The adductor of the Pisces is the mother mass from which the 

 muscles of mastication in front of the quadrate are derived throughout 

 the vertebrates, by the separation of slips of this muscle and by their 

 gradual complete separation in nerve supply through the growth of the 

 originally small twigs into separate nerve branches. In the evolution 

 from fishes this adductor has changed profoundly and much of it has 

 been lost, but the original topographic relations of the muscle remain the 

 same throughout the vertebrates, whether all parts are completely sepa- 

 rated or not. 



(3) Some of the so-called pterygoid muscles have been developed inde- 

 pendently in the amphibians, reptiles and mammals. The new develop- 

 ments have been slips needed in the movements of the pterygoid region 

 in strep tostylic reptiles, and probably a new series of pterygoid muscles 

 for the mammals. 



(4) The amphibians retain rudiments of the levator arcus palatini of 

 the fishes, but these muscles are lost beyond this group. 



(5) In the streptostylic reptiles the complex pterygoid musculature 

 was developed to meet the needs of the movable quadrate. 



(6) The complicated musculature of the pterygoid in the streptostylic 

 forms which was developed by the subdivision of the capiti-mandibularis 

 mass into numerous slips. 



( 7 ) The birds inherit much from the reptiles, and parts of their ptery- 

 goid musculature are undoubtedly homologous with those of reptiles, as 

 follows : 



Pterygoideus profundus of the birds = capiti-mandibularis profundus 

 in the reptiles. 



Pterygoideus anterior of the birds = muscle of the same name in the 

 reptiles. 



The capiti-mandibularis superficialis and capiti-mandibularis medius 

 = subdivided capiti-mandibularis of the reptiles. 



(8) The tensor tympani and the levator veli palati of mammals ap- 

 pear to represent vestiges of the pterygoideus anterior of reptiles. The 

 tensor tympani took on its modern functions when the back part of the 



