Functional lever model of 1st valvifer with both articulations and the beginning of the dorsal ramus of the 1st valvula (arrow) with the intervalvifer articulation acting as pivot point. c = anatomical inlever ; c’ = effective (= mechanical) inlever; d = anatomic outlever; d` = effective (= mechanical) outlever; F(in protraction), F(in retraction) = Force input at the 1st valvifer; F(out protraction), F(out retraction) = Force output at the 1st valvifer that is transferred to the dorsal ramus of the 1st valvula. c’ · F(in) = d’ · F(out). Abbreviations: 1vf = 1st valvifer; dr1 = dorsal ramus of the 1st valvula; iar = interarticular ridge of the 1st valvifer; iva = intervalvifer articulationtva = tergo-valvifer articulation.