16 PROCEEDINGS OP THE JSTATIONAL MUSEUM vol. 67 



were both forced either to the right or to the left. This center is 

 not precisely the anterior one of the three segments of the cranium 

 proper, as has been claimed/" but the ''dead center" may be consid- 

 ered as passing through the frontals above and palatals below. 

 Either of these pairs of bones may vary somewhat in accordance with 

 the portion of the skull either craniad or caudad thereto, according 

 to whether the more powerful influence lies in one direction or the 

 other. In the four skulls examined the original seat of injury has 

 been in the neighborhood of one of the glenoid fossae. 



It is difficult, if not impossible, to speculate with any degree of 

 certainty upon the relative development of the temporals and mas- 

 seters, considered as separate muscles, because of their extreme 

 interdependence. The previous condition of the masseters can only 

 be deduced from the configuration of the zygomatic arch. Reduc- 

 tion in the size of one temporal muscle is not necessarily followed by 

 a smaller zygomatic arch, and therefore by inference, a smaller mas- 

 seter upon that side; but reduction, for any reason, of the size of 

 the arch does seem to result in a lessened volume of the adjacent 

 temporal muscle. 



The interrelationship of the anterior with the posterior portion of 

 the temporal muscle is somewhat obscure, but fluctuations in the size 

 of this muscle are not necessarily uniform for the two parts. The 

 size of the anterior portion of the temporal fossa — lying immediately 

 adjacent to the supraorbital processes in carnivores — may be very 

 much larger, indeed, upon one side when the posterior portion — over- 

 lying the brain case proper — is but a trifle more extensive than upon 

 the opposite side. The explanation of this fact is believed to be 

 that the extreme cranial portion of the temporal muscle is the part 

 that is used in contributing the ultimate contracting power of which 

 the jaw muscles are capable. As this final force can hardly be applied 

 upon the weaker side of the head, because of pain or mechanical 

 disability, nondevelopment of the anterior part of the temporal muscle 

 upon that side of the cranium results. Certain it is that a disparity 

 in the development of the anterior, as compared with the posterior, 

 part of an abnormal temporal fossa recurs sufficiently often to indi- 

 cate a substantial difference in the precise functions of the two 

 corresponding portions of the temporal muscle. 



The pterygoid plates and fossae naturally reflect the development 

 of the pterygoid muscles, and a smaller plate upon one side means 

 that the muscles attached thereto were correspondingly weaker. 



Asymmetrical development of the rear hah of the skull — at least 

 of the superior portion — is closely correlated with size and strength 



"Howell, A. B., Individual and age variation in Microtus montanus yosemite, Journ, Agric. Research, 

 vol. 28, 1924, pp. 977-1016. 



