ADAAIS, PHYLOOENY OF THE JAW MUSCLES 55 



Statement of the Problem 



The problems of the musculature of the jaws and of the homology of 

 these muscles throughout the vertebrates are of great potential impor- 

 tance in the future development of comparative anatomy. As the jaw 

 muscles are concerned in all the movements of the skull and have de- 

 veloped with it, their influence in modifying the skull in the early history 

 of the vertebrates is well worthy of detailed study. The jaw muscles also 

 offer much of interest in their relation to the origin and evolution of the 

 dentition and in the evolution of the mammalian skull from the reptilian 

 type. Such great themes as the origin of the mammalian auditory ossi- 

 cles, the origin of the peculiar mammalian articulation of the mandible 

 with the squamosal, the transformation of the reptilian jaw into the 

 mammalian type, and the homology of certain skull bones in different 

 groups are also involved. The mechanical problems connected with the 

 specialized jaw movements of some of the higher vertebrates are also 

 interesting, especially in cases where the close relation of the jaw muscles 

 and the specialized skull structures are readily discovered. The jaw mus- 

 cles are also sometimes of value in testing relationships of closely related 

 groups, especially where specialization has not gone so far as to obliterate 

 all the primitive conditions, since with a few exceptions the arrangements 

 of the jaw muscles are remarkably constant in fundamental characters 

 in each of the vertebrate classes. 



Criteria of Homology 



The problem of homology in the muscular system is a most difficult 

 one, as the muscles are very unstable elements, given to shifting their 

 positions, splitting up into fragments, and combining with other elements 

 like themselves. 



There are four tests that may be applied to a muscle to determine its 

 possible homology with a similar muscle in another animal: (1) embry- 

 ology, (2) nerve supply, (3) origin and insertion, (4) function. If all 

 of these agree there is great probability that the homology assumed is 

 correct. 



EMBRYOLOGY 



Embryology is helpful in determining origins and in giving a clew, at 

 least, to the relations of the muscle. Keibel and Mall (1910) in their 

 Human Embryology show that primitive relationships are evident in cer- 



