12 MUSEUM BULLETIN NO. 9. 



method of mastication, and carried out with the powerful biting 

 muscles in a high state of contraction. (2) The fact that the 

 whole process is begun at a very early age. This second con- 

 sideration is of great importance, as it is in the young growing 

 skull that environmental reactions produce a lasting effect. 

 Finally, the pressure of the condyle on the eminentia articularis 

 has resulted in the prevention of the downward development 

 of the latter, and has rolled and flattened it out in the manner 

 presented to us so frequently in their skulls. 



In the anthropoid ape the glenoid fossa is shallow and the 

 eminentia articularis flattened (see figures 9 and 10, Plate II), 

 but this cannot be put down to a side-to-side movement in masti- 

 cation, as Dr. Keith has pointed out its impossibility in their 

 case; it may possibly be due to the very heavy mandible and, 

 proportionately to the size of the cranium, huge condyles, 

 combined with extensive forward movements of the condyles 

 in opening and closing the mouth and an antero- posterior 

 movement of the same in mastication. 



To sum up, then, in any primitive race where the food is 

 tough, cookery imperfect, and strenuous side-to-side mastication 

 needed, we should, I think, expect to find examples of shallow 

 glenoid fossae, but in none of them would this be so marked or 

 so universal as in a race such as the Eskimo, living almost 

 exclusively upon a diet of tough and poorly cooked flesh. 



In modern highly civilized man, on the other hand, 

 where, owing to the soft well-cooked nature of the food, 

 such strenuous masticatory movements are no longer necessary, 

 a scissor-like snapping action of the teeth being substituted 

 for the vigorous side-to side grind of primitive man, the condyles 

 in the action of trituration need never move far or with great 

 force out of the glenoid fossae; while, owing to the small extent 

 to which the jaws diverge in the act of mastication, and to the 

 absence of any necessity for powerful biting movements, the 

 condyles of the mandible during mastication need never press 

 with that hard rolling action upon the eminentia articularis. 

 Hence in these races we find a deep glenoid fossa and a high and 

 prominent eminentia articularis. 



