OUR ANCIENT RELATIVES 



of the jaw muscles (the external pterygoid) during 

 the course of its development passed between the 

 lower jaw and its socket in the skull and there gave 

 rise to the bursa or cushion (meniscus); this disc 

 in all typical mammals prevents the lower jaw 

 bone from grinding into its socket in the temporal 

 (squamosal) bone. 



In the immediate ancestors of the mammals the 

 pressure of the dentary bone of the lower jaw 

 transmitted through the meniscus or interarticular 

 disc somehow resulted in the formation of a 

 corresponding socket in the squamosal (temporal) 

 bone of the skull. 



Thus a new or mammalian joint was formed 

 between the dentary bone of the lower jaw and the 

 sJcull, while the old or reptilian joint, lying between 

 the quadrate bone of the upper jaw and the articular 

 bone of the lower jaw, was now greatly reduced in 

 size, continued in the service of the middle ear and 

 gave up its jaw-supporting function. 



These great changes made possible all the new 



lines of evolution of the teeth that the mammals 



developed, which had never been possible for the 



reptiles; with these improved dental equipments 



the mammals soon overran the world, driving out 



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