1879.] 



NATURAL SCIENCES OF PHILADELPHIA. 



51 



and a", the apical portion 

 worn off being restored in 

 outline. This shows that 

 S has been deflected in an 

 inward direction, and indi- 

 cates that the force opera- 

 tive in causing the deflec- 

 tion has been constantly 

 active from one side, or 

 that upon which the large 

 arrow is placed pointing in 

 the direction of the dotted 

 line. The total displacing 

 force exerted by the jaw 

 moving in an inward direction during the entire lifetime of 

 the animal may be supposed to be represented by this arrow, 

 plus that of its ancestry, during the existence of which the tooth 

 has been brought to its present form by the mechanical process 

 of differentiation indicated. The muscular power or crushing 

 force exerted by the mandibular muscles (the coefficient of which 

 is 104 pounds per square inch of section, Haughton) in an upward 

 direction indicated by the lower smaller arrow pointing upwards in 

 thedirection of the entire vertical line, would tend while the mandible 

 is moving inwards and obliquely upwards to act as a wedge upon 

 the somewhat oblique faces of the upper molars, forcing them out- 

 wards in the direction of the dotted line and arrow pointing 

 towards a. In this way the cause of the anisognathism is ac- 

 counted for, whilst it is also not to be forgotten, that the influence 

 of this peculiar combination of forces cannot be without effect in 

 producing modifications upon the crowns of the teeth from within, 

 so that in reality, modifying forces may be at work from both 

 sides. 



