ART. 27 ASYMMETRY IN SKULLS OF MAMMALS HOWELL 7 



An alternative hypothesis, in no wise dependent upon the original 

 injury to the skull, was suggested by Capt. R. W. Leigh (MS), of 

 the Army Medical Museum, to account for the alveolar condition. 

 This is to the effect that the molariform teeth grew into position 

 with abnormally large spaces between them, into which became more 

 or less permanently wedged particles of fibrous foods, this finally forc- 

 ing the recession of the alveolar borders. I have occasionally noted 

 just this state of affairs between two or three teeth of ungulates and 

 rodents, and I deem it very likely that the same situation operated 

 to aggravate the abnormal condition in the gorilla; but that this 

 was the original and sole cause for the recession of the bony borders 

 I strongly doubt. 



An examination of the skull impresses one v/ith the probability 

 that certain muscles suffered considerable violence, either directly or 

 in consequence of severe infection after injury. The evidence indi- 

 cates that the muscles thus involved were chiefly the digastric, tra- 

 chelo-mastoid, sterno and cleido-mastoids, splenius, rectus capitis 

 lateralis, and the levator and tensor palatis. It is not improbable 

 that the obliquus superior was also affected. There is no satisfactory 

 evidence that any other muscle suffered directly except in so far as 

 the pathological condition of the articulation of the right side of the 

 jaw means previous infection, to some extent, of the muscles sur- 

 rounding it. 



The direct results of injury, reflected in the stresses exerted by 

 other muscles of the head, were of a more profo\md and lasting char- 

 acter. Injury to and subsequent healing of a muscle causes, under 

 certain conditions, shrinkage of its length, and other strains may 

 develop. It should here be understood that the term "pull" of a 

 lesioned muscle, "shrinkage," etc., is merely relative and not actual. 

 Shrinkage is very seldom sufficient for a muscle so affected to exert 

 true tension upon a part; but a muscle of this character not only 

 resists full relaxation, but by the same token resists normal growth. 

 Hence, for all practical purposes, it does not matter, when speaking 

 of a case where asymmetry of a bone is due to difference in the 

 development of either of a pair of muscles, whether one conceive 

 that the abnormal side of a bone has been pulled around hj con- 

 traction {i. e., resistance to normal growth), oi' that the growth of 

 the normal side has pushed the other out of line. 



With respect to the long axis of the skull, the foramen magnum 

 is displaced toward the right in such a manner as to suggest that 

 the articulation of the condyle with the atlas was forced in that 

 direction, logically, by the pull of the lesioned obliquus superior and 

 rectus capitus lateralis, and perhaps other, muscles upon that side. 

 The basioccipital naturally is obliged to follow this tendency, and it 



