ABT. 27 ASYMMETRY IN SKULLS OF MAMMALS — HOWELL 17 



of the temporals |and|masseters, but such a condition of the face," 

 when marked, can not be due primarily to myological stimuli. It 

 may be attributable first to a setting out of plane, through nongrowth 

 of one side, of the bones with which the rostrum articulates, as a 

 house may be thrown out of plumb by the settling of one side of its 

 foundation; or it may be due to the pull, through the interlocking 

 of the canines, primarily exerted by a glenoid fossa, and hence the 

 mandible, which has been displaced. 



Asymmetry of the occipital plane may be ascribable in part to 

 differences in the size of the lambdoidal crests induced either by the 

 temporal muscles, or by variation in certain of the cervical muscles, 

 or both. As only the insertions, and not the origins, of the latter 

 are available for examination, interpretation of the few facts presented 

 is difficult. 



The form of an asymmetrical mandible is determined mostly by 

 the positions of the glenoid fossae at one extreme, and by the force 

 that may be exerted by the interlocking canines at the other. Vari- 

 ation in size between the mandibular processes of the two rami is 

 not likely to be as great as are certain asymmetrical differences in 

 the skull proper. In other words, the muscular origins seem to be 

 more sensitive to asymmetrical influences than are their insertions. 

 Asymmetry of the mandible, in fact, seems to be due chiefly to 

 mechanical stimuli. 



11 For convenience the posterior portions of the zygomatic processes of themaxillse are here considered 

 as not belonging to the face proper. 



