ASYMMETRY IN THE SKULLS OF MAMMALS 



By A. Brazier Howell 



OJ the Bureau of Biological Survey, United States Department of Agriculture 



Reference to moderate asymmetry in human skulls is not infre- 

 quent throughout medical literature, but marked asymmetry in the 

 crania of the other mammals (save the toothed whales, in which this 

 state is the normal one) must be considered as a very rare condition. 

 Careful examination of the material in any la.rge collection woiild 

 doubtless result in the discovery of a number of specimens showing 

 some disparity between the development of the two sides of the 

 skull; but it is exceedingly seldom that one occurs in which such 

 condition is readily to be noted. Application to those in charge of 

 some of the larger mammal collections of North America have pro- 

 duced but four specimens, two of which exhibit more emphatic dis- 

 tortion than seems ever to have been recorded. 



Asymmetry in a skull may be brought about by a change in the 

 size or relationship, through a,ccident or disease, of the individual 

 bones of one side of the head, an alteration, through the same agency, 

 to one or more of the large muscles upon a single side, or a combi- 

 nation of these two factors. A further analysis of these reasons 

 will suggest, as fundamental causes, a more or less permanently pain- 

 ful condition of some part of the head, as a sore tooth, a diseased, 

 mandibular condyle, or other such state causing the animal to chew 

 entirely upon the teeth of one side, or otherwise to use the muscles 

 in an uneven manner so as to ease the pain of the offending part as 

 much as possible. This at first is voluntary, although it may later 

 become entirely involuntary, and it implies a long-continued period 

 of painfulness of the part originally affected. Again there may be 

 a definite alteration in the shape of a bone, through fracture and 

 later healing in a twisted position, changing the interrelationship of 

 other parts of the skull, as the mandible. Accidental severance of 

 certain nerves may also be productive of similar results. In all such 

 cases of asymmetry in the skull, initial injury at a comparatively 

 early age is a necessity— the earlier the injury the more pronounced 

 will be its effects, otlier things being equal. 



The pathological conditions resulting from the healing of a severe 

 injury to the bone after the animal has attained full growth does not 



No. 2599.— Proceedings u. S. National Museum, Vol. 67, Art. 27. 



63194— 26t — 1 1 



