Instead of a unitary field of visibility and order, whose elements have a distinctive value in relation to each other, we have a series of oppositions, of which the two terms are never on the same level: on the one hand, there are the secondary organs, which are visible on the surface of the body and offer themselves without intervention to immediate perception, and, on the other, the primary organs, which are essential, central, hidden, and unreachable except by dissection that is, by materially removing the coloured envelope formed by the secondary organs.