Sentence:  [W]hen a proportionable piece [of an aliment (food item)] is thus taken into the Mouth, the lower Jaw is variouſly mov'd by its proper Muſcles, and Maſtication is begun and carryed on by the Aſſiſtance of the Tongue, Cheeks, and Lips; the two firſt ſtill applying the leſs divided parts of the Aliment, to the "Dentes Molares" [molar teeth], till there's an equal <WSD>Comminution</WSD> of all its Parts; ... 
Meaning of the word marked with <WSD> tags:  The breaking or grinding up of a material to form smaller particles.