THE HUMAN RACHIS: CAN IT BE CONSIDERED A SHOCK ABSORBER (THAT WAS PRODUCED BY EXAPTATION) RATHER THAN A COLUMN?

Our paper is focused on two fundamental points: the first one is a terminological proposal and the second one is a question. Obviously these two things are strictly related one another. The terminological proposal is aimed to name “vertebral shock absorber” the human rachis (globally considered, when it is in physiological conditions) in its most typical function: to sustain static/dynamic stresses, that moreover are directed according to its axial direction, obviously when this coincides with gravitational line. This aspects can be studied by modal analysis and by the model of Eigenvectors and Eigenvalues. According to our opinion, the mechanical feature must be considered as prevalent if compared with the structural one. Following it human rachis is usually named “column”. This mechanic sustain is distributed on three lines that are summarily parallel and are linked one another (by isthmuses and vertebral arches to build an horizontal ring) so they can be considered a unique compact viscous-elastic system. Each one of these three vertical sub-structure is built as a stacking of metameric elements (modules) along a continuous line.