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Rh were it not for those curves and the elasticity of the intervertebral cartilages. To use a familiar simile, we may liken the head to that of a hammer, which is placed on a straight stick or handle. Every one knows that the surest way to fasten the head is to knock the handle on the ground, as the concussion drives it further in; so that if the human head were carried on a straight support, similar concussions would be caused to the brain and other vital organs, to their serious if not fatal injury. Of course extra muscular strength is required to support the increase of weight caused by those curves; and this is effected by an extra stress on the muscles of the back. "The waving line of the column arising from a series of alternate curves in opposite direc­tions is altogether peculiar to man; it allows a proper distribution of the weight with respect to the centre of gravity, the line of which, carried through the entire trunk, must fall within the entire space covered by the feet, or by one foot when we support the body by one only. As this line passes through all the curves, motion is allowed in the upper regions without impairing the general equili­brium."