AKT. 27 ASYMMETRY IX SKULLS OF MAMMALS HOWELL 13 



The above theoretical deflection of the symphysis by 20 mm. is 

 purely mechanical in character. All the remaining forces of asym- 

 metry that have operated to alter the skull are either myological 

 or developmental (growth of bone). 



An examination of the superior aspect of the skull indicates that 

 in comparison with normal crania the one under discussion seems to 

 be unusually small, as well as can be judged from general criteria 

 of age. Not only is it short, but the sagittal crest is more poorly 

 developed. The cephalic musculature was therefore undoubtedly 

 below normal, although one may presume that it was entirely ade- 

 quate for the feeding needs of the animal. 



The sagittal crest is displaced about 10 mm. toward the left, but 

 with this exception there is no decided indication that the posterior 

 portion of the right temporal muscle was more powerful than that 

 upon the left. There is striking evidence to this effect, however, 

 in the anterior part of the right temporal fossa. As clearly shown 

 in the illustration, this portion of the right temporal was several 

 times the larger, not only displacing the sagittal crest but extending 

 well forward upon the frontal and modifying the shape and size of 

 the right supraorbital process. The sinistral displacement of the 

 rostrum is largely attributable to the disproportionately great 

 development of this part of the muscle, coupled with the fact that the 

 right zygomatic arch and masseter were free to accomplish anterior 

 growth. On the other hand the stunting of the left zj^gomatic arch 

 resisted normal growth of the left haK of the skull, and necessarily 

 limited the size of the mass of muscle that could pass within the 

 zygomatic vacuity of the temporal fossa. These would seem to be 

 the two causes that tended to limit the growth to the left temporal 

 muscle, while the reduced length of the zygomatic arch upon that 

 side prevented normal growth, and undoubtedly strength, of its 

 attached masseter. Thus, during the development of the animal, 

 the root or base of the left side of the rostrum was held back, while 

 that of the right side was pushed forward, resulting in the great dis- 

 placement toward the left exhibited by that part of the skull. 

 Another consequence has been the disproportionate, dextral bowing of 

 the mesethmoid, and undoubtedly of the cartilaginous septum, which 

 resulted in a crowding of the right ethmoid and, therefore, expansion 

 of the left nasal passage, from which the ethmoid is now missing. 

 This whole process has naturally effected a disproportionate devel- 

 opment of the individual bones of the rostrum which is of interest; 

 but there is no necessity here for dwelling at greater length upon 

 their individualities. 



Returning to an examination of the mandible, one notes that there 

 is no indication of asymmetry in its muscular insertions. It is clear, 

 however, that the displacement of 20 mm., caused by the shifting 



