ART. 27 ASYMMETRY IN SKULLS OF MAMMALS HOWELL 15 



condition of the cervical region; but all such hypotheses are pure 

 speculation. 



CONCLUSIONS 



From the study of these four specimens it has been concluded 

 that the primary cause inducing asymmetry in the skulls of mam- 

 mals other than toothed cetaceans is probably, in most instances, by 

 accident or disease, to the bones or muscles of a single side of the 

 head at a comparatively early age, and that this must be of such a 

 character as to result in a stunted or infantile condition of a crucial 

 part of the bony framework, and a reduction in the rate of growth, 

 or strength through lesions, of the muscles of a single side. Asym- 

 metry usually is directly dependent upon unevenness in the strains 

 developed upon the two sides of the head while an animal is eating. 



Certain injury to the bones of the head causes a premature oblit- 

 eration of the sutures, as already indicated. Published data respect- 

 mg human crania have shown that such early obliteration of the 

 sutm*es may also occur from obscure causes without violence having 

 been suffered by the individual. It is doubtless fortuitous that no 

 skulls of this character have been available in the present study. Con- 

 versely, it is known that retarded obliteration of certain sutjiires be- 

 yond the usual time for their disappearance results in the hypertrophy 

 of the corresponding part of the skull. It is only a question of time 

 before material illustrating the latter point in the mammalia other 

 than man is brought to light. 



One of the most conspicuous results of this investigation, and one 

 that deserves to be stressed, is the conclusion that normal develop- 

 ment of the bones of the skull is directly dependent upon the growth 

 of the attached muscles. In other words, if for any reason the mus- 

 cles of an animal remain infantile and fail properly to grow, the bones 

 to which they are secured will remain proportionately undersized. 

 This assertion can not be proven in the case of the masseter muscles 

 until the myology of asymmetrical skulls can be more fully investi- 

 gated. It is also apparent that the smaller the origin or fossa of a 

 muscle the smaller must the muscle itself be." The significance of 

 these facts, when considered with reference to specific (and higher) 

 variation of the skull, is profound. 



It is apparent that asymmetrical development of a skull inclines 

 to progress both forward and backward from a center that is rather 

 uniform. In other words, it always appears as though a part of the 

 skull were held stationary while the portions craniad and caudad 



•Of interest in this connection is a paper by J. A. Howell (An experimental study of the effect of stress 

 and strain on bone development, Anat. Rec, vol. 13, 1917, pp. 233-252) comparing the leg bones of a dog, 

 the muscles upon one side of which had been transected when the animal was very young. The diameter 

 of the bone was very greatly diminished thereby, but its length was but little below normal. 



