ADAMS, PHYLOGENY OF THE JAW MUSCLES 103 



against the pull of the temporal, which, together with the pull of the 

 pterygoid muscles, gives the steadiness that is necessary in the mandibles. 

 In many cases there is a connection of the masseter with the auditory 

 region. In Tamandua (Lubosch, 1908, p. 541) this small muscle is 

 seen, as it were, in the process of splitting off from the masseter and 

 attaching itself to the tympanic. Bradypus villosus (Lubosch) shows 

 this same condition, where a small muscle extends from the angle of the 

 jaw to the tympanic. This small muscle is innervated, according to 

 Lubosch, by the auriculo-temporalis nerve. The tendency of the masseter 

 to extend to the auditory region probably may go back to the reptilian 

 condition, where this muscle was attached to the quadrate or to one of 

 the bones in this region. 



The pterygoid muscles arise on the basicranial or pterygoid region and, 

 although usually small, they are of importance in the working of the 

 jaws. They vary in form in different groups and the relation between 

 the pterygoids and the dentition is very close. They are largely devel- 

 oped in herbivorous but small in carnivorous mammals. They function 

 in giving the jaw steadiness in opening and closing; they prevent the 

 heavy temporal muscles from tearing the mandibles apart in forms with 

 a loose symphysis and they make possible the grinding motion of the 

 teeth in herbivorous forms. The edentates and monotremes offer excep- 

 tional conditions, for here the reduced or modified condition of the man- 

 dible makes the pterygoid muscle of little importance. 



The internal pterygoid is the more important of the two, as it aids in 

 the closing of the jaw and pulls inward against the pull of the temporal, 

 masseter, etc. The external pterygoid pulls the jaw forward, out of the 

 glenoid cavity, and assists in this way in opening the jaw and in the 

 forward movement of the jaw in herbivorous forms. 



The origins of these muscles are plainly seen in mammalian skulls and 

 the condition of the pter} T goid region has probably been very much modi- 

 fied by their action, as a comparison with the reptilian condition shows 

 that the pterygoid region is pinched in and pulled to the rear. We assume 

 that this condition has been brought about largely by the action of ptery- 

 goid muscles. In cynodonts the opposite pterygoid bones meet in the 

 mid line, but in mammals the pterygoids have become much reduced and 

 separated so as to leave the primary floor of the brain case exposed (basi- 

 sphenoid, presphenoid) (Gregory and Adams, 1915). 



The capiti-mandibularis profundus (external pterygoid) of the reptiles 

 seems to be in a favorable position to give rise to the whole pterygoid 

 mass of the mammals, since it is inserted on the coronoid region of the 

 mandible, and might easily shift its insertion down on the inner side of 



