With the help of a simple geometric construction, it is possible to demonstrate an indication given by Rudolf Steiner that the skull bone is an inversion of a long bone. Inversion means that a form can be turned inside out and Vice versa. The construction comprises moving all points of a form by a constant distance (inversion constant) through a single point (inversion point). 50 that a true inversion can take place, the inversion point must lie inside the basic form and all points must pass through the inversion point. In the inversion, the inside of the basic form reproduces itself between the inverted borderline of the basic form and the circumcircle. The circumcircle is the circle round the inversion point with a radius corresponding to the size of the inversion constant.